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.
This first pass only applies to the following two cases:
- Public functions returning a view type into an object they own
- Public ctors storing a view type
This catches a grand total of one (1) issue, which is fixed in
the previous commit.
