Add exponentiation methods for BoxedUint

[andrewwhitehead]

This implements wrapping_pow, wrapping_pow_bounded_exp, wrapped_pow_vartime for BoxedUint, as well as checked and saturating versions of these methods.

Multiplication support (references to the karatsuba module and overflow checks) are moved into UintRef.

A BoxedMultiplier helper is added for reducing allocations during the exponentiation.

Related to #259

[codecov]

Codecov Report

❌ Patch coverage is 99.73615% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.13%. Comparing base (f25910b) to head (d8bb0c1).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/uint/boxed/mul.rs	98.18%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1159      +/-   ##
==========================================
+ Coverage   80.66%   81.13%   +0.47%     
==========================================
  Files         178      181       +3     
  Lines       18963    19277     +314     
==========================================
+ Hits        15296    15641     +345     
+ Misses       3667     3636      -31     

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[andrewwhitehead]

@tarcieri I just noticed that (&BoxedUint) * (&BoxedUint) was doing a checked multiplication, unlike all the other Mul implementations so I changed it to be consistent and do a widening multiplication.

[tarcieri]

Wonder if there's a way we can consolidate this with e.g. BoxedMontyMultiplier