Fix undefined constant expression evaluation

[jcdickinson]

This Pull Request fixes/closes #644 and #631.

It changes the following:

• Adds support for undefined expr
• Makes the Node match exhaustive (I have ConditionalOp and Continue as a local WIP, but they don't really belong in this PR)

[codecov]

Codecov Report

Merging #645 into master will increase coverage by 0.00%.
The diff coverage is 85.71%.

@@           Coverage Diff           @@
##           master     #645   +/-   ##
=======================================
  Coverage   72.94%   72.95%           
=======================================
  Files         178      178           
  Lines       13234    13240    +6     
=======================================
+ Hits         9654     9659    +5     
- Misses       3580     3581    +1     

Impacted Files	Coverage Δ
boa/src/exec/mod.rs	70.80% <0.00%> (-0.45%)	⬇️
boa/src/exec/tests.rs	100.00% <100.00%> (ø)

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[Razican]

Thanks! Looks good from my side :)