Problem:
- The Base64 alphabet and lookup table are initialized at
run-time. This results in an initial start-up cost as well as a
boolean evaluation and branch every time the function is called.
Solution:
- Provide `constexpr` functions which initialize the alphabet and
lookup table at compile-time. These can be called and assigned to a
`constexpr` variable so that there is no run-time cost associated
with the initialization or lookup.
This enables a nice warning in case a function becomes dead code. Also, add forgotten
header to Base64.cpp, which would cause an issue later when we enable -Wmissing-declarations.
