[expr] "raise to power" operator, similar to [pow] function

Returns an integer value if both arguments are integers, and [double]-precision floating-point otherwise.

The left operand may be any integer from Tcl's unlimited integer range.

The right operand is limited to a max integer value of 268435455 (0xfffffff) (28 bits).
(Unless the left operand is -1, 0, or 1, so that the answer is trivial).

Expect to wait a long while for [expr 2**0xfffffff] to return.