[wasm-c-api] Implement missing functions (#1461)
[wasm-c-api] Implement missing functions (#1461)

For now, the wasm-c-api interface in wabt doesn't have its own tests,
and relies on the wasm-c-api repo's examples for testing. This means
that some functions that are not used in any examples were accidentally
omitted.

TODO: wabt (or wasm-c-api) should have explicit tests of the API
surface.
WebAssembly/sexpr-wasm-prototype,WebAssembly/sexpr-wasm-prototype,WebAssembly/sexpr-wasm-prototype,WebAssembly/wabt,WebAssembly/wabt,WebAssembly/wabt,WebAssembly/sexpr-wasm-prototype,WebAssembly/wabt,WebAssembly/sexpr-wasm-prototype,WebAssembly/sexpr-wasm-prototype,WebAssembly/wabt