Add checked_pow function

[est31]

Implements a checked_pow function which does the same as pow, just with overflow checks.

And, similar to #152 and #153, the function uses references instead of cloning.

Adds a little macro to spare code repetition. Its scoped to the function so nothing gets polluted.

[cuviper]

Looks good -- it's nice that you mimicked pow so closely.
@homu r+ 3c5ecdc

[homu]

⚡ Test exempted - status