Add support for value assignment in declaration

[cburgdorf]

What was wrong?

Previously, this code would fail:

another_reference: u256[10] = my_array

As a workaround declaration and assignment could be split apart.

another_reference: u256[10]
another_reference =  = my_array

With this change, the shorter declaration with assignment syntax is supported.

How was it fixed?

• Added implementation to map value assignments in declarations
• Rewrote test code that used workaround

To-Do

• OPTIONAL: Update Spec if applicable

• Add entry to the release notes (may forgo for trivial changes)

• Clean up commit history

[codecov-io]

Codecov Report

Merging #173 (d8b0837) into master (3826aea) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #173      +/-   ##
==========================================
+ Coverage   79.98%   80.01%   +0.03%     
==========================================
  Files          44       44              
  Lines        3198     3198              
==========================================
+ Hits         2558     2559       +1     
+ Misses        640      639       -1     

Impacted Files	Coverage Δ
compiler/src/yul/mappers/declarations.rs	100.00% <ø> (ø)
compiler/src/abi/elements.rs	74.64% <0.00%> (+1.40%)	⬆️

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