BitField: Add an explicit evaluation method.

Sometimes it can be beneficial to use this in places where an explicit cast needs to happen otherwise. By using the evaluation method, it's not necessary anymore to explicitly write the underlying type in this case.
BitField: Delete copy assignment to prevent obscure bugs.

Cf. https://github.com/dolphin-emu/dolphin/pull/483
BitField: Add a static_assert.

Being able to store BitField within unions requires BitField to be of standard layout, which in turn is only given if the underlying type is also has standard layout.
BitField: Cast enum values to proper integer type.
Merge pull request #30 from neobrain/bitfield_work

BitField additions and fixes