(improvement) Allow for concurrent multi-threaded building of Runes

[Nathan13888]

With some Rune files using models with many labels, the build time seems to almost exponentially increase in time complexity.

Currently rune build seems to be single-threaded. Making the build process multi-threaded would allow leveraging high core count systems (like those found in servers).

Not sure of the exact solution as I can't find much information about multi-threaded builds for wasm, so it may be less trivial than it seems.

If there is a solution for this, perhaps a new flag like -c <threads> or -t <threads> would be intuitive for specifying the number of threads to use.

[Michael-F-Bryan]

I think you might be running into #177 (comment) here.

The issue isn't that we don't build Runes in parallel (parallelism gives linear speedups while this seems to be quadratic/exponential, anyway) but that the code we generate hits an edge case in the Rust compiler which causes pathological build times.

This should be resolved by #310 because you'll be passing a single (albeit long) string literal to the label proc block.

[Michael-F-Bryan]

Closing as a duplicate of #177.