Add wasm2c tool (#710)

Add `wasm2c`, a new tool that reads a `.wasm` file and generates a C
source file and its accompanying header file. The C output currently
only supports gcc/clang compilers, since it uses builtins for some
functionality.

The resulting C code is not standalone; there are runtime functions that
must be provided, as well as pointers to all imports.

The C runtime symbols that must be provided are as follows:

* `void wasm_rt_trap(wasm_rt_trap_t code)`:
  Called when the WebAssembly code traps. This function must not return.
* `u32 wasm_rt_register_func_type(u32 param_count, u32 result_count, ...)`:
  Register a function type with the given signature. This function must
  check whether this signature has already been registered and return
  the original index.
* `void wasm_rt_allocate_memory(wasm_rt_memory_t*, u32 initial, u32 max)`:
  Allocate the memory buffer for the given memory object, given the
  number of pages. The memory must be zeroed before returning.
* `u32 wasm_rt_grow_memory(wasm_rt_memory_t*, u32 delta)`:
  Grow memory by the given number of pages. If allocation fails, or the
  new pages size is larger than the maximum, return -1. Otherwise return
  the previous number of pages. The newly allocated memory must be
  zeroed.
* `void wasm_rt_allocate_table(wasm_rt_table_t*, u32 initial, u32 max)`:
  Allocate the buffer for the given table object. The buffer must be
  zeroed before returning.
* `u32 wasm_rt_call_stack_depth`:
  A symbol that tracks the current call stack depth. If this value
  exceeds `WASM_RT_MAX_CALL_STACK_DEPTH` then a trap occurs. This value
  defaults to 500, but can redefined.

An example implementation can be found in `spec-wasm2c-prefix.c`.

All functionality from the WebAssembly MVP is supported, and the
generated code passes all of the core spec tests. There is a new test
tool called `run-spec-wasm2c.py` which runs the following:

* `wast2json` to convert the spec test to json and wasm files
* `wasm2c` to convert the wasm to C source and headers
* a C compiler (default `cc`) to compile and link all C source files,
  including a C test runner (`spec-wasm2c-prefix.c`)
* Finally, the resulting executable to produce output

1 files changed, 1 insertions, 0 deletions

diff --git a/README.md b/README.md
index da85e01c..e978419e 100644
--- a/README.md
+++ b/README.md
@@ -10,6 +10,7 @@ WABT (we pronounce it "wabbit") is a suite of tools for WebAssembly, including:
  - **wasm-interp**: decode and run a WebAssembly binary file using a stack-based interpreter
  - **wat-desugar**: parse .wat text form as supported by the spec interpreter (s-expressions, flat syntax, or mixed) and print "canonical" flat format
  - **wasm-link**: simple linker for merging multiple wasm files.
+ - **wasm2c**: convert a WebAssembly binary file to a C source and header
 
 These tools are intended for use in (or for development of) toolchains or other
 systems that want to manipulate WebAssembly files. Unlike the WebAssembly spec