Turn _idn2_punycode_encode, _idn2_punycode_decode into compat symbols

These internal symbols should not be used by applications, but were previously exported.

The aliases are required because internal hidden references turn the target symbol into a hidden symbol. It would have been possible to preserve the internal API using another asm alias, but this commit renames the call sites instead, for simplicity.

Fixes #63 (closed).