* `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`.
