Add support for module names in wasm binaries

[dschuff]

Basically implement WebAssembly/design#1055. We want binary parser and output support for it, and also asm2wasm/s2wasm support, which IMO should be independent of other debug info so that we can just use it all the time in emscripten (i.e. modules always have module names even if they have no function or local names). The cost of that will be very low, and it will be very useful for debugging and stack traces, especially along with WebAssembly/design#1053

[dschuff]

cc @jgravelle-google and @kripken in case I don't get to it.

[jfbastien]

Looks like you didn't get to it?