parsers: stop Choice from looping on a nested choice parser

[rameel]

Choice flattens nested choices to avoid extra parser calls. The flattening loop appended parsers to a list without rebinding the input array, so a nested Choice never let the loop terminate:

using static Ramstack.Parsing.Parser;

var inner = Choice(L('a'), L('b').Between(L('('), L(')')));
var outer = Choice(inner, L('c')); // out of memory

The fix expands nested unnamed choices in one pass. A choice is flattened when it is created, so one pass expands the whole tree.

A named nested choice stays a single alternative and keeps its name.