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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
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