Add enum StrSlice under enum Evaluated

[zmrdltl]

Description

after #966
If a function that handles a string is chained and executed on the query builder, the string is copied every time by the Clone trait. This overhead is solved by always having a sliced ​​enum with a value.

Implementation plan

#[derive(Clone)]
pub enum Evaluated<'a> {
    Literal(Literal<'a>),
    StrSlice { source: String, range: Range<usize> },
    Value(Value),
}

If a variant called source holds the source of the string, and the string function is called continuously after that, the range is updated every time.

Tasks

• add enum StrSlice

modify string function structure for StrSlice

• • Substr
• • trim
• • ltrim
• • rtrim

• tests

[coveralls]

Pull Request Test Coverage Report for Build 4096482561

• 827 of 827 (100.0%) changed or added relevant lines in 18 files are covered.
• 64 unchanged lines in 7 files lost coverage.
• Overall coverage decreased (-0.05%) to 98.513%

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Files with Coverage Reduction	New Missed Lines	%
core/src/ast/mod.rs	1	99.8%
storages/sled-storage/src/index_mut.rs	1	98.66%
core/src/translate/mod.rs	4	95.63%
core/src/executor/alter/table.rs	6	94.92%
storages/sled-storage/src/alter_table.rs	14	96.13%
core/src/executor/execute.rs	19	91.77%
test-suite/src/tester/mod.rs	19	87.33%

Totals
Change from base Build 4040662072:	-0.05%
Covered Lines:	38951
Relevant Lines:	39539

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💛 - Coveralls

[zmrdltl]

Failed to write part 2 test.

[panarch]

Awesome!! New big improvement comes into the project.
Thanks a lot 👍 👍 👍