Implement pow/pow_with_context (exponentiation with integer parameter)

[drinkcat]

pow: Fix nonstd build, and build with older versions

Directly build stdlib::num::NonZeroU64 with new instead of using
try_into, stub out println! in tests.

Also avoid using ilog2 and div_ceil.

pow: More analytical thinking about additional margin

It does appear adding an additional digit per ten multiplications
might be enough. At least this seems to be enough to not be
able to find any counter example within reasonable time.

pow: Add a little bit of additional margin

For unclear reasons related to rounding, we still sometimes get
the last digit wrong. Adding a little bit of extra margin fixes
the problem (or at least, makes it extremely unlikely to happen).

pow: Allow negative exponents

Supporting negative exponents is fairly straightforward, we
just need to be careful to start with enough precision when doing
the initial invertion.

pow: Add pow/pow_with_context function

Add a function to raise a BigDecimal to an integer power.

The number of digits would blow up if we kept the exact value, so
we use the precision provided in Context parameter (or the default
value).

We need to keep additional precision during intermediate
computations though, as each multiplication may lose up to 2 digits.

For tests, we compare against values that we can obtain from
Wolfram Alpha, and test a few cases with 50 or 500 precision context,
to check that the precision loss compensation work we do works
correctly.

For now, only support positive exponent values.

Fixes #147.

[drinkcat]

Fixed the CI, updated margin logic to add multiplications/10 as extra margin (not that I'm fully convinced by the argument in #147...)

Still some rare errors with this logic.

[akubera]

Finally released in 0.4.9. Thanks.