(PUP-957) Remove special meaning of Regexp[r] in =~ !~ expressions.

There are several issues with the previously implemented support
where a parameterized Regexp was a stand in for a real regular
expression:

• The match variables where not set (and would be difficult to return in general since Regexp's and Patterns can be nested in other types).
• Blurry line between what a Regexp type is, and String since Regexp[r]

produces a RegularExpression Type, not a Regexp instance.

This simply removes the special handling. A user can still use:

• Regexp (/..../)
• A String - transforms to a Regexp
• Pattern[p] type (since it is a real subtype of String)