implement named args

[metagn]

closes #522

When encountering a named argument in a call, sem semchecks its value and passes it along with the name to sigmatch. It is also tracked if a named argument was given, in which case the arguments are reordered before passing to the sigmatch proc.

The way the reordering is done: The signature params are iterated over first to build a table of names to param positions etc. Then the given arguments are iterated over, saving the positions of arguments belonging to each parameter. Then a new list of arguments is built in the order of the parameters. Varargs is considered by saving arguments as "continued", i.e. the parameter position is not advanced for them when building the list. Missing parameters add a dot token to mark a default value, but this does not fail early to behave the same as missing default values in normal matching.

A quirk of original Nim is that positional arguments are not influenced by named arguments before them, i.e. in foo(name1 = val1, name2 = val2, val3, ...), val3 always matches the 3rd parameter of the proc. This behavior is preserved here.