Use template specializations to avoid shift warnings.

[LTLA]

May (or may not) address the segfault observed in #15.

[rstub]

It feels weird having to define these template specializations even though they will never be called. I tried to get rid of the compiler warnings using tag dispatch on shiftable. But the there was lots of code duplication between the true and falsecases.

[rstub]

BTW, since we know that these four template specializations will never be called, can you please add // # nocov to the end of those lines? that way the error from the codecov check should go away.