Instead of having all primitives and constructions share the same namespace,
they are now organized by category and function family.
Types within the same category are expected to share the exact same API.
* remove deprecated `std.fs.Dir` APIs
* `std.fs.Dir.openDir` now takes a options struct with bool fields for
`access_sub_paths` and `iterate`. It's now much more clear how
opening directories works.
* fixed the std lib and various zig code calling the wrong openDir
function.
* the runtime safety check for dir flags is removed in favor of the
cheaper option of putting a comment on the same line as handling
EBADF / ACCESS_DENIED, since that will show up in stack traces.
* `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`.
