Shrink the stack a bit by putting LetData in Box. Accomodates things…

[prozacchiwawa]

… needed by the assign and lambda prs.

[coveralls-official]

Pull Request Test Coverage Report for Build 5871628343

• 6 of 7 (85.71%) changed or added relevant lines in 4 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 78.484%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/compiler/evaluate.rs	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/compiler/sexp.rs	1	78.85%

Totals
Change from base Build 5869378500:	0.1%
Covered Lines:	7299
Relevant Lines:	9300

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💛 - Coveralls