Allows different implementations for different platforms (the JS
build relies on browser APIs to stream disk images over the network).
Setting aside the JS build, this also reduces some code duplication.
Result of running IWYU (https://include-what-you-use.org/) and
applying most of the suggestions about unncessary includes and
forward declarations.
Was motivated by observing that <thread> was being included in
ppcopcodes.cpp even though it was unused (found while researching
the use of threads), but seems generally good to help with build
times and correctness.
