Add structured trees to preprocessor

[matt-o-how]

Converts #(a b c) into (a b . c)

[coveralls-official]

Pull Request Test Coverage Report for Build 6084706666

• 42 of 49 (85.71%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 79.824%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/compiler/sexp.rs	42	49	85.71%

Files with Coverage Reduction	New Missed Lines	%
src/compiler/sexp.rs	1	75.43%

Totals
Change from base Build 6028353730:	0.03%
Covered Lines:	11426
Relevant Lines:	14314

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💛 - Coveralls

[prozacchiwawa]

Neat.