Add array/slice Limb => Word cast helpers

[tarcieri]

Adds the following functions which can cast from arrays or slices of Limb to Word:

• Limb::array_as_words
• Limb::array_as_mut_words
• Limb::slice_as_words
• Limb::slice_as_mut_words

[codecov]

Codecov Report

❌ Patch coverage is 70.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.26%. Comparing base (75e45d5) to head (86ad81e).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/limb.rs	68.75%	5 Missing ⚠️
src/uint/ref_type.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1103   +/-   ##
=======================================
  Coverage   79.26%   79.26%           
=======================================
  Files         177      177           
  Lines       18076    18088   +12     
=======================================
+ Hits        14328    14338   +10     
- Misses       3748     3750    +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.