Introduce Word256 integer type

[darkdrag00nv2]

Closes #2389
Work towards onflow/developer-grants#179

Description

This PR introduces the type Word256 to Cadence.

Will soon send the documentation PR along with Word128.

---

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[codecov]

Codecov Report

Merging #2510 (06fa3ac) into master (326312c) will decrease coverage by 0.11%.
The diff coverage is 63.53%.

@@            Coverage Diff             @@
##           master    #2510      +/-   ##
==========================================
- Coverage   78.45%   78.35%   -0.11%     
==========================================
  Files         336      336              
  Lines       76561    77078     +517     
==========================================
+ Hits        60068    60395     +327     
- Misses      14262    14427     +165     
- Partials     2231     2256      +25     

Flag	Coverage Δ
unittests	78.35% <63.53%> (-0.11%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
encoding/ccf/traverse_value.go	98.30% <ø> (ø)
runtime/interpreter/hashablevalue.go	100.00% <ø> (ø)
runtime/interpreter/primitivestatictype_string.go	100.00% <ø> (ø)
runtime/interpreter/visitor.go	8.33% <0.00%> (-0.18%)	⬇️
runtime/interpreter/value.go	68.67% <54.51%> (-0.41%)	⬇️
runtime/interpreter/encode.go	67.26% <66.66%> (-0.01%)	⬇️
types.go	85.54% <66.66%> (-0.13%)	⬇️
encoding/json/decode.go	88.02% <68.75%> (-0.31%)	⬇️
runtime/interpreter/decode.go	46.88% <70.58%> (+0.26%)	⬆️
values.go	76.19% <80.00%> (+0.10%)	⬆️
... and 13 more

[turbolent]

Nice!