refactor: Switch serde dependency to serde_core

[jplatte]

Improve build parallelism :)

[epage]

Please keep in mind that I prefer a clean commit history and am perfectly fine with editing of commits

[coveralls]

Pull Request Test Coverage Report for Build 17735157779

Details

• 17 of 52 (32.69%) changed or added relevant lines in 11 files are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.007%) to 70.535%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/toml/src/ser/value/array.rs	3	5	60.0%
crates/toml_edit/src/ser/array.rs	4	6	66.67%
crates/toml/src/ser/document/array_of_tables.rs	0	4	0.0%
crates/toml/src/de/deserializer/key.rs	0	6	0.0%
crates/toml_edit/src/de/key.rs	0	6	0.0%
crates/toml_datetime/src/datetime.rs	4	11	36.36%
crates/serde_spanned/src/spanned.rs	0	8	0.0%

Files with Coverage Reduction	New Missed Lines	%
crates/toml_edit/src/de/key.rs	3	45.0%
crates/toml/src/de/deserializer/key.rs	3	43.24%

Totals
Change from base Build 17160864059:	-0.007%
Covered Lines:	6820
Relevant Lines:	9669

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💛 - Coveralls

[epage]

Thanks!