Faster tests through heap images

[mbrock]

We should be able to generate a basic stdlib image via build.zig before running the test suite, so each test can load the image instead of loading the whole stdlib.

[mbrock]

Got quite far with this today but I implemented loading with readv calls on file input streams, which won't work for embedded constant data in the executable, and that's really the more important use case, so I should just read the data from a slice reader instead.

• [x] rewrite Tape.load to read from memory instead of a file