feat: Allow indented continuation lines

Resolves #2.

tests/basic.spt

@@ -14,7 +14,8 @@ 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 Receives(Stdio(), Sum([Int("72"), Int("87"), Int("33")]))
+]]/* **/ parse to TopLevel(Receives(Stdio(),
+                                    Sum([Int("72"), Int("87"), Int("33")])))
 /** writes
 192**/
 
@@ -62,9 +63,9 @@ left-associative, so that way we can chain insertions into a stream:
 
 /** md */ test emit_twice [[
 stdio << 72 + 87 + 33 << 291
-]]/* **/ parse to Receives(
-  Receives(Stdio(), Sum([Int("72"), Int("87"), Int("33")])),
-  Int("291"))
+]]/* **/ parse to TopLevel(
+  Receives(Receives(Stdio(), Sum([Int("72"), Int("87"), Int("33")])),
+           Int("291")))
 /** writes
 192291**/
 
@@ -80,25 +81,48 @@ with `>>`; both forms return the first argument:
 
 /** md */ test enters_twice [[
 (7 + 8 >> stdio + 9) >> stdio
-]]/* **/ parse to
-  Enters(Sum([Int("7"), Enters(Int("8"), Stdio()), Int("9")]), Stdio())
+]]/* **/ parse to TopLevel(
+  Enters(Sum([Int("7"), Enters(Int("8"), Stdio()), Int("9")]), Stdio()))
 /** writes
 824**/
 
+/** md
+```
+
+### Layout in fostr
+
+Expressions may be laid out onto multiple lines, as long as all continuation
+lines are indented from the start of the initial line:
+```fostr
+**/
+
+/** md */ test receive_enter_break [[
+stdio <<
+  7
+  + 8 >> stdio
+  + 9
+]]/* **/ parse to TopLevel(
+  Receives(Stdio(), Sum([Int("7"), Enters(Int("8"), Stdio()), Int("9")])))
+/** writes
+824**/
+
+/** md
+```
+(So for example you will get a parse error with something like this:)
+```fostr
+**/
+
+/** md */ test enter_receive_bad_break [[
+(7 + 8 >> stdio + 9)
+>> (stdio << 9 + 2)
+]] /* **/ parse fails
+
 /* Extra tests not in the tour */
-
-test receive_enter [[
-stdio << (7 + 8 >> stdio + 9)
-]]/* **/ parse to
-  Receives(Stdio(), Sum([Int("7"), Enters(Int("8"), Stdio()), Int("9")]))
-/** writes
-824**/
-
 test enter_receive [[
 (7 + 8 >> stdio + 9) >> (stdio << 9 + 2)
-]]/* **/ parse to
+]]/* **/ parse to TopLevel(
   Enters(Sum([Int("7"),Enters(Int("8"),Stdio()),Int("9")]),
-         Receives(Stdio(),Sum([Int("9"),Int("2")])))
+         Receives(Stdio(),Sum([Int("9"),Int("2")]))))
 /** writes
 81124**/