Make strong typedefs able to be constexpr

[drewr95]

I noticed that the strong typedefs _ETLTYPEDEF doesn't use constexpr when I think it could. I think this could additionally be applied to the custom enum macro and user type macros as well.

Example:

ETL_TYPEDEF(float, seconds);
constexpr seconds duration{1.0F};

Trying to constexpr a strong typedef results in a compilation error:

error: constexpr variable cannot have non-literal type 'const seconds' (aka 'const type_def<seconds_tag, float>')

[jwellbelove]

Fixed 20.38.11