Allow "replace" function in Calc to have missing or empty replacement value

[rlwhitcomb]

... meaning to remove it, as in:

a = 'abc'
replace(a, 'c') -> "ab"

This is more complicated syntax because there is also the optional mode parameter.

[rlwhitcomb]

The syntax that works the best is to require the extra comma if the replacement value is missing and the mode is used, as in:

replace 'abcabc', 'b', , first -> "acabc"
replace 'abcabc', 'b' -> "acac"

[rlwhitcomb]

Worse problem is the ambiguity when doing:

replace(read(file), 'a', 'b')

where the "read" greedily takes two params, leaving two for "replace", when it should be "read" has one, and "replace" has three.