Emit component name in core Wasm file names

[guybedford]

This updates the generator to rather than outputting module{n}.wasm for the core Wasm files, to instead use the naming scheme {name}.core{n}.wasm and also in addition when n == 0 to emit the n.

This way applications with a single Wasm file get an unnumbered Wasm, and the loaded Wasm file has a component-specific name eg in the network tab etc.

In addition this also fixes a one-liner regression in the Node.js compat output which wasn't using compileStreaming, and corrects the unhandled rejection handler attachment in the singular case for the JS generator.

[alexcrichton]

Could this perhaps be threaded through to the generators in some form of a structure describing the names? That should avoid having three locations that all need to be kept in sync as the naming scheme gets fancier.

[guybedford]

Good point, I've updated the PR to use a core_file_name trait function.