* Use std.Build.Cache.Directory instead of a string for storing the
cache roots and build roots.
* Set up a std.Build.Cache in build_runner.zig and use it in
std.Build.RunStep for avoiding redundant work.
This provides file path as a command line argument to the command being
run, and returns a FileSource which can be used as inputs to other APIs
throughout the build system.
Unfortunately, it is implemented by pooping a ton of temporary files
into zig-cache/tmp for the time being. I think one of the very next
improvements to the build system should be moving the compiler's cache
system to the standard library and using it in the build system. I had a
look at the dependencies and it is already pretty untangled.
Usage of `catch unreachable` in build scripts is completely harmless
because build scripts are always run in Debug mode, however, it sets a
poor example for beginners to learn from.
