Enable String#to_i spec

[xulaus]

Relates to #1912

Currently String#to_i isn't in the enforced specs, and there deviations from MRI because of this.
This PR makes to_i spec compliant, asides from the cases where a Bignum is needed which now throw a NotImplemented error instead.

While doing this work I did attempt to use the scolapasta-int-parse implementation, however as to_i parses as much as possible before failure and scolapasta either passes or returns an error I would have had to make more changes that I felt comfortable with in that library to make the specs pass. This has lead to a duplication of logic, but there is already an issue to fix this (#2075)

[lopopolo]

This is looking good so far and I'm excited to turn the specs on. I think there are some spots where this code still drifts from MRI.

For the cases that are not covered by the specs, can you add some integrations tests to:

artichoke/artichoke-backend/src/extn/core/string/string_test.rb

Lines 5 to 16 in 255dd13

	def spec
	string_match_operator
	string_element_reference_regexp
	string_byteslice
	string_scan
	string_unary_minus
	string_reverse
	string_tr
	string_end_with
	true
	end

[lopopolo]

this looks great! thank you for the logic fixes, the new specs, and the new integration tests