Allow MappedFile to expose a byte range within an fd. It maps the
page-aligned region required by mmap internally.
Add ImmutableBytes as a small shared holder for either owned bytes or a
mapped file. This lets cache code thread file-backed response bodies
through without copying response data into anonymous memory.
Cover non-page-aligned ranges, empty ranges, invalid ranges, and mapped
ImmutableBytes in TestLibCoreMappedFile.
