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.
These tests previously only ran on SerenityOS. They needed test input
location changes. The Stream tests also needed to explicitly set
SO_REUSEADDR for the tcp servers.
Let's replace this bool with an `enum class` in order to enhance
readability. This is done by repurposing `MappedFile`'s `OpenMode` into
a shared `enum` simply called `Mode`.
The internal reuse of FixedMemoryStream makes this straightforward.
There alread is one user of the new API, demonstrating the need for this
change beyond what I said out to use it for :^)
