Consider alternative implementations

[propensive]

Several of the original uses of Contextual have been subsequently replaced with macros which operate directly on the StringContext and parts values. The motivation for using Contextual was that macros are hard to use, and they could be avoided. But the use of interpolator objects forces a more complex model for each implementation, which really isn't necessary if you understand macros.

So, we should entertain the idea of chucking out the Interpolator code, and just making it easier to implement each macro as per #78.