Move formatters etc. into own file + tests

All tests pass and building, using the client works as expected.

The `Format__MP` tests should cover almost everything, tests for `strToInt64` were copied from zathras, `rounduint64` has none.

`strprintf` was an alias for `tfm::format` in [util.h](https://github.com/dexX7/bitcoin/blob/mcore-formatters-and-tests/src/util.h#L116) which refers to [tinyformat.h](https://github.com/dexX7/bitcoin/blob/mcore-formatters-and-tests/src/tinyformat.h) which seems to have *many* boost dependencies.

I'm not sure about function names. Upper case, lower case, ...?

Also note: `FormatDivisibleMP` and `FormatIndivisibleMP` were in namespace `mastercore`. If they should be in an explicit mastercore namespace, then I suggest to remove the MP suffix.