Initial pass parsing/reading struct (#1352)

This parses just the format `(struct)` as a new type. I added a test for
this using `wat2wasm`, but that requires a rudimentary binary format.
The test runner automatically attempts to rountrip all wat2wasm tests,
so this required implementing the wat writing and binary reading too.

Here's a summary of the changes:

* binary-reader:h: Rename `BinaryReader::OnType` callbacks to `OnFuncType`
* binary-reader.h: Add `BinaryReader::OnStructType`
* binary-reader.cc: Use a switch after reading the type form to
    determine whether we're reading a function or struct.
* tokens.def: Add new `TokenType::Struct`
* lexer-keywords.txt: Add new `struct` keyword
* type.h: Add `Type::Struct` type form
* wast-parser.cc: Parse `(struct)` in text format
* wat-writer.cc: Write Func or Struct type forms

1 files changed, 6 insertions, 5 deletions

diff --git a/src/binary-reader-logging.h b/src/binary-reader-logging.h
index facc8d5d..214d8bae 100644
--- a/src/binary-reader-logging.h
+++ b/src/binary-reader-logging.h
@@ -42,11 +42,12 @@ class BinaryReaderLogging : public BinaryReaderDelegate {
 
   Result BeginTypeSection(Offset size) override;
   Result OnTypeCount(Index count) override;
-  Result OnType(Index index,
-                Index param_count,
-                Type* param_types,
-                Index result_count,
-                Type* result_types) override;
+  Result OnFuncType(Index index,
+                    Index param_count,
+                    Type* param_types,
+                    Index result_count,
+                    Type* result_types) override;
+  Result OnStructType(Index index) override;
   Result EndTypeSection() override;
 
   Result BeginImportSection(Offset size) override;