Boxed Query body to save some stack space

[5tan]

Boxing Query::body reduced size of Query from 1312B to 592B.

In following test of standalone parser runs (7MB stack) allowed subquery nesting jumped from 239 to 286.

x=286 ; echo 'print("SELECT * FROM {}ttt{}".format("("*'$x',")"*'$x'))' | python3 > test.sql && cargo run --example cli --manifest-path Cargo.toml test.sql --snowflake

This makes significant difference when parsing "recursive queries" in environments with reduced stack size (e.g. like tokio spawned threads - 2MB).

[alamb]

Thanks @5tan -- I think this change looks reasonable to me. For what it is worth, I found that significant difference in rust when compiling with --release -- apache/datafusion#1047 for details

[coveralls]

Pull Request Test Coverage Report for Build 2663763457

• 16 of 16 (100.0%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 89.872%

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Files with Coverage Reduction	New Missed Lines	%
src/parser.rs	1	83.36%

Totals
Change from base Build 2631347546:	-0.02%
Covered Lines:	8865
Relevant Lines:	9864

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💛 - Coveralls