feat: sequencing of expressions with newline to same indent

Also revised README to reflect greater emphasis on streams.

  Resolves #3.

tests/basic.spt

@@ -13,9 +13,9 @@ that writes the sum of the ASCII codes for 'H', 'W', and '!' to standard output:
 **/
 
 /** md */ test emit_sum [[
-stdio << 72 + 87 + 33
-]]/* **/ parse to TopLevel(Receives(Stdio(),
-                                    Sum([Int("72"), Int("87"), Int("33")])))
+stream << 72 + 87 + 33
+]]/* **/ parse to TopLevel(Gets(Stream(),
+                                Sum(Sum(Int("72"), Int("87")), Int("33"))))
 /** writes
 192**/
 
@@ -40,32 +40,27 @@ For example, this snippet generates the following Python:
   start: 'Stdio\s='
 !}
 ```
-(which writes "192" to standard output), or this non-idiomatic, inefficient, but
-working Javascript:
-```javascript
-{! ../tests/emit_sum.js extract:
-  start: '^}'
-!}
-```
-In either case, there's also a preamble defining Stdio that's generated.
-(Haskell code generation is also currently supported.)
+(which writes "192" to standard output); it also generates identical code in
+this simple example for
+Javascript, although it generates a different preamble defining Stdio in each
+case. (Haskell code generation is also currently supported.)
 
 ### Everything has a value
 
 As mentioned in the [Introduction](../README.md), everything in a fostr
 program (including the entire program itself) is an expression and has
-a value. So what's the value of that expression above? Well, `stdio` is our
+a value. So what's the value of that expression above? Well, appropriately
+enough, `stream` is our
 first example of a stream, and for convenience, the value of a stream
-receiving an item is just the stream back again. The `<<` operator is also
-left-associative, so that way we can chain insertions into a stream:
+receiving an item is (usually) just the stream back again. The `<<` operator
+is also left-associative, so that way we can chain insertions into a stream:
 ```fostr
 **/
 
 /** md */ test emit_twice [[
-stdio << 72 + 87 + 33 << 291
+stream << 72 + 87 + 33 << 291
 ]]/* **/ parse to TopLevel(
-  Receives(Receives(Stdio(), Sum([Int("72"), Int("87"), Int("33")])),
-           Int("291")))
+  Gets(Gets(Stream(), Sum(Sum(Int("72"), Int("87")), Int("33"))), Int("291")))
 /** writes
 192291**/
 
@@ -75,14 +70,14 @@ Running this program produces a nice palindromic output: "192291".
 
 And because sometimes you want to emphasize the value and propagate that
 instead of the stream, you can also write these expressions "the other way"
-with `>>`; both forms return the first argument:
+with `>>`; both forms return the first argument, so the following writes "824":
 ```fostr
 **/
 
 /** md */ test enters_twice [[
-(7 + 8 >> stdio + 9) >> stdio
+(7 + 8 >> stream + 9) >> stream
 ]]/* **/ parse to TopLevel(
-  Enters(Sum([Int("7"), Enters(Int("8"), Stdio()), Int("9")]), Stdio()))
+  To(Sum(Sum(Int("7"), To(Int("8"), Stream())), Int("9")), Stream()))
 /** writes
 824**/
 
@@ -97,12 +92,12 @@ lines are indented from the start of the initial line:
 **/
 
 /** md */ test receive_enter_break [[
-stdio <<
+stream <<
   7
-  + 8 >> stdio
+  + 8 >> stream
   + 9
 ]]/* **/ parse to TopLevel(
-  Receives(Stdio(), Sum([Int("7"), Enters(Int("8"), Stdio()), Int("9")])))
+  Gets(Stream(), Sum(Sum(Int("7"), To(Int("8"), Stream())), Int("9"))))
 /** writes
 824**/
 
@@ -113,19 +108,56 @@ stdio <<
 **/
 
 /** md */ test enter_receive_bad_break [[
-(7 + 8 >> stdio + 9)
->> (stdio << 9 + 2)
+(7 + 8 >> stream + 9)
+>> (stream << 9 + 2)
 ]] /* **/ parse fails
 
 /* Extra tests not in the tour */
 test enter_receive [[
-(7 + 8 >> stdio + 9) >> (stdio << 9 + 2)
+(7 + 8 >> stream + 9) >> (stream << 9 + 2)
 ]]/* **/ parse to TopLevel(
-  Enters(Sum([Int("7"),Enters(Int("8"),Stdio()),Int("9")]),
-         Receives(Stdio(),Sum([Int("9"),Int("2")]))))
+  To(Sum(Sum(Int("7"),To(Int("8"),Stream())),Int("9")),
+     Gets(Stream(),Sum(Int("9"),Int("2")))))
 /** writes
 81124**/
 
+/** md
+```
+
+Of course, fostr programs are not limited to one line; expressions on successive
+lines are evaluated in sequence. For example, the program
+```fostr
+**/
+
+/** md */ test emit_thrice [[
+  stream << 72 + 87
+  stream << 88
+    + 96
+  99 + 12 >>
+          stream
+]] /* **/ parse to TopLevel( Sequence(
+    [ Gets(Stream(), Sum(Int("72"), Int("87")))
+    , Gets(Stream(), Sum(Int("88"), Int("96")))
+    , Sum(Int("99"), To(Int("12"), Stream()))]))
+/** writes
+15918412**/
+
+/** md
+```
+
+will write 15918412. fostr enforces that successive expressions in sequence
+must line up at the left, i.e., the following will not parse:
+
+```fostr
+**/
+
+/** md */ test emit_thrice_bad_alignment [[
+  stream << 72 + 87
+stream << 88
+    + 96
+  99 + 12 >> stream
+]] /* **/ parse fails
+
 /** md
 ```
 **/

tests/emit_sum.fos

@@ -1 +1 @@
-stdio << 72 + 87 + 33
+stream << 72 + 87 + 33

tests/emit_thrice.fos

@@ -0,0 +1,5 @@
+  stream << 72 + 87
+  stream << 88
+    + 96
+  99 + 12 >>
+    stream