Add internal identity mappings.

This is done to get range checking even when the type can't take on
out-of-range values.  This typically draws a warning "comparison is
always 0 due to limited range of data type"; we want the tests for
future-proofing but don't want the warnings.  But we want limited-range
warnings turned on in general.

Simply using a cast doesn't defeat the warning.  So we pass such values
through an identity-mapping function which widens them in a way that
doesn't provoke the warnings.

I'm willing to accept the extra cost of calling the trivial function
for the sake of future-proofing the range checks.