The goal is to forward-declare utils::multiprecision_int, something
beyond my capabilities for boost::multiprecision::cpp_int, to reduce
compile time bloat.
The patch is mostly search-and-replace, with a few casts added to
disambiguate conversions the compiler had trouble with.
1. Move tests to test (using singular seems to be a convention
in the rest of the code base)
2. Move boost tests to test/boost, other
(non-boost) unit tests to test/unit, tests which are
expected to be run manually to test/manual.
Update configure.py and test.py with new paths to tests.
