Preserve regex captures across stack frames

[johnkerl]

Regex captures for the =~ operator should belong to stack frames at the user-defined-function level.

Example:

mlr -n put '
func f() {
    if ("456 defg" =~ "([0-9]+) ([a-z]+)") {
        print "INNER: \1 \2";
    }
}
end {
    if ("123 abc" =~ "([0-9]+) ([a-z]+)") {
        print "OUTER PRE:  \1 \2";
        f();
        print "OUTER POST: \1 \2";
    }
}'

Before:

OUTER PRE:  123 abc
INNER: 456 defg
OUTER POST: 456 defg

After:

OUTER PRE:  123 abc
INNER: 456 defg
OUTER POST: 123 abc

To do: unit-test cases, and doc updates.

Context: second PR on #1401.