fix Issue 12197 - 'invalid array operation' errror message with valid array operation

Make explicit importing std.math unnecessary for pow expressions.

Currently front-end does not enforce std.math module existence. Therefore,
there's not also necessity to explicitly import std.math in user code.
.
Merge pull request #3304 from 9rnsr/implicit_pow

[enh] Make explicit importing std.math unnecessary for pow expressions