For enums, use to_string, from_string. [Depends on TS 21.]

[mgates3]

Depends on icl-utk-edu/testsweeper#21

Use standard to_string function to convert enum to string.
Introduce to_char and to_c_string functions.
Introduce from_string function to convert string to enum.
This also makes the ParamEnum constructor much simpler.

[mgates3]

@weslleyspereira Any thoughts on this PR? Would this affect <T>LAPACK or other things you work on?

Basically, this replaces enum-specific functions like uplo2str and str2uplo with overloaded versions like to_string and from_string. Since you can't overload on the return type, from_string takes a dummy argument to specify the return type. TestSweeper no longer needs to be told what a parameter's input & output functions are (e.g., uplo2str, str2uplo), it can just assume to_string and from_string.

[weslleyspereira]

@weslleyspereira Any thoughts on this PR? Would this affect <T>LAPACK or other things you work on?

I think it is a good idea! Thanks for tagging me. I don't think it will change anything in <T>LAPACK.