[flang][runtime] Support internal I/O to CHARACTER(KIND/=1)

Allow internal I/O to support non-default kinds of CHARACTER.

The I/O runtime design anticipated this standard feature, but
this patch is somewhat larger than I thought it would be because
many code sites had to have assumptions about units (characters
vs. bytes) brought into harmony, and some encoding utilities
had to be pulled out of IoStatementState and templatized into
their own new header file so that they are available to formatted
output code without having to "thread" an IoStatementState reference
through many call chains.

Differential Revision: https://reviews.llvm.org/D131107