* Step.init() now takes an options struct
* Step.init() now captures a small stack trace and stores it in the
Step so that it can be accessed when printing user-friendly debugging
information, including the lines of code that created the step in
question.
And additionally support writing files to source files. This means a
custom build step in zig's own build.zig is no longer needed for copying
zig.h because it is handled by WriteFileStep.
* 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.
Breaking API change to std.Build.addConfigHeader. It now uses an options
struct.
Introduce std.Build.CompileStep.installConfigHeader which also accepts
an options struct. This is used to add a generated config file into the
set of installed header files for a particular compilation artifact.
std.Build.ConfigHeaderStep now additionally supports a "blank" style
where a header is generated from scratch. It no longer exposes
`output_dir`. Instead it exposes a FileSource via `output_file`.
It now additionally accepts an `include_path` option which affects the
include path of CompileStep when using the `#include` directive, as well
as affecting the default installation subdirectory for header
installation purposes.
The hash used for the directory to store the generated config file now
includes the contents of the generated file. This fixes possible race
conditions when generating multiple header files simultaneously.
The values hash table is now an array hash map, to preserve order for
the "blank" use case.
I also took the opportunity to remove output_dir from TranslateCStep and
WriteFileStep. This is technically a breaking change, but it was always
naughty to access these fields.
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.
