For example, when only a single file (or stdin) is being searched, then we
should be able to print directly to the terminal instead of intermediate
buffers. (The buffers are only necessary for parallelism.)
Closes#4.
I though plain `read` had usurped them, but when searching a very small
number of files, mmaps can be around 20% faster on Linux. It'd be really
unfortunate to leave that on the table.
Mmap searching doesn't support contexts yet, but we probably don't really
care. And duplicating that logic doesn't sound fun. Without contexts, mmap
searching is delightfully simple.
