Workaround for #34 (#35)

This PR adds a workaround for the case where a global symbol is matched instead of the local `error`/`value` template in `valueOr` and friends, when `valueOr` is being used in a generic context.

Two options are added:
* when using Nim version supporting the experimental `genericsOpenSym` [feature](nim-lang/Nim#23873), we use it
* when not, a macro tries to replace all accesses in the given body with the correct symbol - this matching is done by name and some minimal heuristics which potentially could be wrong - the upstream solution is obviously preferable

Both solutions can be disabled via compile-time options to retain the old behavior of matching the global symbol.

See also nim-lang/Nim#22605

thanks to @Araq for the macro hack and @metagn for the language fix!

Author: arnetheduck