This moves the CONST replacement up before we turn the elements into a
single string. The advantage is that we don't have to worry about
splitting the string properly to find the CONST names as the elements
are already split for us.t
Currently functions only have very limited optional argument support,
this expands it so that we can have more complex sets of optional
arguments for functions, such as multiple arguments where not all need
to be provided. This will be used in the future for some upcoming
functionality.
Note that this does not support any generic optional argument format,
the commas always have to be provided unless an optional argument is at
the end of the parameter list. Thus, if you have a format with two
optional arguments and you want to omit the second, then you need to call
it as 'foo(2, , 3)`, rather than `foo(2, 3)`. This is important for
avoiding ambiguous situations, and is how many SUBs currently function.
The two functions that violate that requirement are INSTR() and
_INSTRREV(), which use the format `[?],?,?` and allow omitting the comma
for the first argument. This format is simply handled as a special case.
Fixes: #303
