Limit casemapping to appropriate ranges in ERC

Limit casemapping to appropriate ranges in ERC

* lisp/erc/erc-common.el (erc-downcase): Use case table for
`erc-downcase' so that case conversions are limited to the ASCII
interval.
* lisp/erc/erc.el (erc-casemapping--rfc1459-strict,
erc--casemapping-rfc1459): Make these case tables instead of
translation tables.  The functions in case-table.el modify the
standard syntax table, but that doesn't seem to make sense here,
right?
* test/lisp/erc/erc-tests.el (erc-downcase): Add cases showing
mappings outside of the ASCII range.  (Bug#59976.)