Add PAIR! support for REPEAT

This change allows PAIR! values as value argument of REPEAT. This
enables convenient traversal over a 2D space:

    >> repeat p 3x3 [print p]
    1x1
    2x1
    3x1
    1x2
    2x2
    3x2
    1x3
    2x3
    3x3

Similar to REPEAT over decimals, REPEAT over pairs implies a start of
1.0x1.0 and a default bump of 1.0x1.0. The first pair component ("X") is
varied first, which is in line with pair!-based indexing into image!s.

For the sake of completeness, PAIR! support is also enabled in the
legacy C-style FOR:

    >> for p 10x10 15x20 5x5 [print p]
    10x10
    15x10
    10x15
    15x15
    10x20
    15x20

    >> for p 3x1 1x2 -1x1 [print p]
    3x1
    2x1
    1x1
    3x2
    2x2
    1x2

This implements [CureCode wish #1995](http://issue.cc/r3/1995).