Fix bugs in CSE Machine hanging during array access

[alsonleej]

Description

Resolves #1749 and #1751 and completes #1740

Type of change

• Bug fix (non-breaking change which fixes an issue)

now gives an error message instead of hanging for

• trying to access array when LHS is not array
• trying to spread array when RHS is not array

prevents crashing for

• trying to access out of bounds array

Checklist

• I have tested this code
• I have updated the documentation

[coveralls]

Pull Request Test Coverage Report for Build 14197561406

Details

• 19 of 21 (90.48%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 81.11%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/cse-machine/interpreter.ts	14	16	87.5%

Totals
Change from base Build 13964710825:	0.01%
Covered Lines:	10803
Relevant Lines:	12949

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💛 - Coveralls

[martin-henz]

Please add test cases to get good coverage of this fix.

[martin-henz]

Quite a few warnings in src/cse-machine/interpreter.ts
Please take a look.

[alsonleej]

Bugfixes / enhancements:

1. CSE Machine now shows error for accessing array with a) negative index and b) index > 2^31 -1, instead of simply crashing. For indices below 2^31 -1 but still out of range, the CSE Machine pushes an undefined to the stash. This is as per Source 3 specification.

2. When testing it appears that no undefined is pushed to the stash, but it actually is, and it can be viewed by skipping to the last step. The animation for it is completed by CSE Machine: Added animation for spread instruction frontend#3114 , so when both changes are merged, the full effect is complete.

3. Added test cases for spread operator, along with array access.

[arnav-goel10]

LGTM!