feat: Implement enters operator >> (#8)

Also adds parenthesization of fostr expressions.
  Recasts code generation in terms of bottomup processing
  of a local strategy.

  Resolves #1.

Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Reviewed-on: #8
Co-Authored-By: Glen Whitney <glen@nobody@nowhere.net>
Co-Committed-By: Glen Whitney <glen@nobody@nowhere.net>

tests/basic.spt

@@ -71,4 +71,37 @@ stdio << 72 + 87 + 33 << 291
 /** md
 ```
 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:
+```fostr
+**/
+
+/** md */ test enters_twice [[
+(7 + 8 >> stdio + 9) >> stdio
+]]/* **/ parse to
+  Enters(Sum([Int("7"), Enters(Int("8"), Stdio()), Int("9")]), Stdio())
+/** writes
+824**/
+
+/* 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
+  Enters(Sum([Int("7"),Enters(Int("8"),Stdio()),Int("9")]),
+         Receives(Stdio(),Sum([Int("9"),Int("2")])))
+/** writes
+81124**/
+
+/** md
+```
 **/