Updated CONTRIBUTING.md to help people get started

Not doing so can cause errors while interacting with external code if
such code is executed bet...

Otherwise subclasses would have tried to call accessors on themselves,
resulting in all nil valu...

ref: https://github.com/ariya/phantomjs/issues/12697

String#gsub fully compliant with rubyspec

Fix incorrect MatchData#pre_match and #post_match behavior

String#sub fully compliant with rubyspec

String#rindex fully compliant with rubyspec

Back to sprockets ✨

The amount of FP here probably calls out for a new class with its own
private methods.

* Funct...

Instead of a complex ERB rendering of a constant with current `binding`.

Sometimes files stubbed in opal don’t exist at all, but sprockets would
require them to be there...

That was copied as is from the erb tilt template.

RFC: String#casecmp fully compliant with rubyspec

Get rid of most of the String#each_line and String#lines failures

String "spring cleaning" continues: partition, rpartition, *, +, new, upto, sum, tr and tr_s.

String#delete implemented

String#count fully compliant with rubyspec

Engines are classes and not instances, `<=` will check for subclasses including self.

String#squeeze fully compliant with rubyspec

Struct#values_at fully compliant with rubyspec

Struct#to_h implementation compliant with rubyspec

Which has been removed in sprockets v3

Update spec/corelib rubyspec submodule repo to latest commit

String bugs cleanup, passing #strip, #rstrip, and #hex.

Struct#[] and Struct#[]= fully compliant with rubyspec

Update spec/corelib submodule to make String#===, String#==, and String#eql? fully compliant wit...

This makes them available externally (specifically to opal-rails).

Should be sprockets and not sprockets.cache.