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, 3 insertions, 0 deletions

diff --git a/test/parse/module/struct.txt b/test/parse/module/struct.txt
new file mode 100644
index 00000000..e199a0ce
--- /dev/null
+++ b/test/parse/module/struct.txt
@@ -0,0 +1,3 @@
+;;; TOOL: wat2wasm
+;;; ARGS: --enable-gc
+(type (struct))