gen-host-js: --load flag support

[guybedford]

This unifies the core Wasm loading in instance mode under a single load model controlled by a --load flag which defaults to --load=fetch. The reason for this was when working on creating asm.js output for the core Wasm binaries, this required a lot of custom code splicing which could be avoided by just having well-defined loading functions that can be overriddent when under a --load=custom mode.

There are two loading functions - loadWasm and instantiateWasm.

instantiateWasm is always just declared as const instantiateWasm = WebAssembly.instantiate, unless using --load=custom which then will not define either loadWasm or instantiateWasm at all, and allow the user to define these functions via importing / concatenation or whatever means is suitable.

The --base64 option is then made available as --load=base64.

The nice thing about this structure is it opens up future load modes, like a --load=reflect for if/when import reflection progresses (working on it!).

Note the driving use case for this work was very much supporting asm.js component generation, and I have verified this all works correctly with loadWasm effectively being a noop and instantiateWasm wrapping the asm.js wiring.