Replace macros with associated type, add more descriptive names

[sufferiing]

I have replaced the uses of the TYPENAMES and DIMS macros with an associated type Quantity::Self, as well as gave some of the internal components more descriptive names. This should hopefully be a step towards making the errors generated by this library a bit more comprehensible. In a future PR I will work on adding static_asserts as well.

I also fixed some minor bugs in the units versions of the <math> functions.

[Aang099]

Have you run this with an example program through Godbolt or similar to make sure compiled code behavior is consistent? I don't know specifically why not but C++ has surprised us before.

[sufferiing]

I did now, the compiled code is identical.

[Aang099]

LGTM then