Make prefixLength of MultilineStringValueParser a method

[ehmry]

This fixes parsing of multiline strings that contain whitespace.

I was digging around in spryextend because I would like to make an extension for Preserves data. Would it be feasible to add additional composite types that are homoiconic? I'd like to add Preserves records which are a labeled tuple type. I'm experimented with <'foo 1 2 3> and <<'foo 1 2 3>> syntax but I'm worried that nested records are going to be a problem for the parser. FWIW there are Preserves libraries for Smalltalk and Nim so I think Spry should be doable (I already have a module, but it's not homoiconic).

[ehmry]

Maybe I can have custom type that $s to 'foo record: [ 1 2 3 ] to represent <foo 1 2 3> if I have a nimMeth of record: to construct it?