--usenimcache (implied by nim r main) now caches some compile options to avoid recompiling when project was previously compiled with such options.

[timotheecour]

this PR improves the compilation cache, and in particular nim r, --usenimcache, -d:nimbetterrun

--usenimcache (implied by nim r main) now generates an output file that includes a hash of some of the compilation options, which allows caching generated binaries

example

nim r main # recompiles
nim r -d:foo main # recompiles
nim r main # uses cached binary
nim r main arg1 arg2 # ditto (runtime arguments are irrelevant)

see tests

before this PR

nim r main # recompiles
nim r -d:foo main # recompiles
nim r main # bad: recompiles (after this PR, does not recompile)
nim r sub/main # recompiles
nim r main # bad: recompiles (after this PR, does not recompile)

notes

• follows new cmd: nim r main [args...] to compile & run, saving binary under $nimcache/main #13382
• this feature exists in D with rdmd
• fixes nim r -d:foo pathto/main should cache all CT options in filename to avoid recompiling when not needed timotheecour/Nim#698
• closes --usenimcache:usehash timotheecour/Nim#199
• see also Add a 'run' command to zig? ziglang/zig#466, Add run compiler command ziglang/zig#874
• fixes most of [tooling] cache compilation to avoid recompiling when possible RFCs#107

future work

• (preexisting limitation) nimbetterrun (and writeJsonBuildInstructions etc) isn't supported with js backend, but there's not reason it shouldn't
• writeJsonBuildInstructions + friends should just reuse std/json, it'll simplify code. simpler = better.
• [tooling] cache compilation to avoid recompiling when possible RFCs#107 meantions another useful features that would be nice to implement: garbage collection, using an LRU cache of cached binaries