zig build system: organize build artifacts

[andrewrk]

migrated from #204

• have a "build" directory where all build artifacts go
• ability to override default zig directory with --build-dir [name]
• use a unique filename for linker script and delete it on success
• for compiling C files, mirror source directory structure so that the object file paths can be the same

[andrewrk]

Currently, all intermediate artifacts go into the cache dir, and final artifacts go into the cwd.

I think how the rest of these are going to work, is that even the final build artifacts go into the cache dir. If you want build artifacts such as executables and libraries to go somewhere else, you can use the setOutputPath API, or you can run the install step.

Note that the cache dir is going to change structure with #330 and #298.