* `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`.
Widening and truncating integer casting to different signedness
works better now. For example `(unsigned long)-1` is now translated
to zig code that compiles correctly.
Block-local identifiers have block-local mangling numbers, and more
consistent mangling is applied within blocks. Parameters, for example,
are treated the same as other block-local variables, and are not mangled
unless they conflict with another name in scope.
According to C11 6.5.6.8, pointer arithmetic may not overflow. In fact,
it may not even go more than 1 past the end of an object, or UB occurs.
This is the same as Zig pointer arithmetic semantics, and so the
`+` and `+=` operators rather than `+%` and `+%=` are appropriate for
C-translated pointer arithmetic.
