Generate _tp_impl for externals with varargs and use _tp_impl functions in `usesymbols`

Changes:

• importsymbols No need to import impl if type patterns are disabled.
• usesymbols If type patterns enabled, make both the function itself and the impl variant available.
• type_pattern_resolve If an external function has varargs (...), simply duplicate it and rename it to the impl function for now. We do this because we always assume that both foo and _tp_impl_foo are available if type patterns are enabled. Handling these cases requires passing of ... or __va_args__ in the SaC world, which is non-trivial.
• rterror No point in inlining conditionalAbort, this is not performance-critical anyways but would produce a lot of inlined code.