backport: fs wrapper

[patricklodder]

This backports the fs wrapper facility from Bitcoin Core that enables us to do a number of future enhancements, like allowing windows datadir paths in different codepages and allowing a means to ease a switch between boost::filesystem and std::filesystem (with c++17).

Backported commits:

• 19e36bbe
• 2a5f5747
• 75594bd7

Manually executed:

• replacing all boost/filesystem header includes with fs.h
• replacing all boost::filesystem:: calls with fs::
• removing file-local fs namespace aliases
• additional implementation of fsbridge::fopen in bitcoin-tx.cpp and test/script_tests.cpp

This is split off from #3229 and a prerequisite for that.

[chromatic]

Concept ACK, I like what I see on a skim. It'll take some time to get to this in detail, but I'm +1 so far.

[chromatic]

All tests pass on x86-64 Linux, so I'll review the commits next.