rework and improve some of the zig build steps

 * `RunStep` moved to lib/std/build/run.zig and gains ability to compare
   output and exit code against expected values. Multiple redundant
   locations in the test harness code are replaced to use `RunStep`.
 * `WriteFileStep` moved to lib/std/build/write_file.zig and gains
   ability to write more than one file into the cache directory, for
   when the files need to be relative to each other. This makes
   usage of `WriteFileStep` no longer problematic when parallelizing
   zig build.
 * Added `CheckFileStep`, which can be used to validate that the output
   of another step produced a valid file. Multiple redundant locations
   in the test harness code are replaced to use `CheckFileStep`.
 * Added `TranslateCStep`. This exposes `zig translate-c` to the build
   system, which is likely to be rarely useful by most Zig users;
   however Zig's own test suite uses it both for translate-c tests and
   for run-translated-c tests.
 * Refactored ad-hoc code to handle source files coming from multiple
   kinds of sources, into `std.build.FileSource`.
 * Added `std.build.Builder.addExecutableFromWriteFileStep`.
 * Added `std.build.Builder.addExecutableSource`.
 * Added `std.build.Builder.addWriteFiles`.
 * Added `std.build.Builder.addTranslateC`.
 * Added `std.build.LibExeObjStep.addCSourceFileSource`.
 * Added `std.build.LibExeObjStep.addAssemblyFileFromWriteFileStep`.
 * Added `std.build.LibExeObjStep.addAssemblyFileSource`.
 * Exposed `std.fs.base64_encoder`.

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