Introduce fpath-sexp0 - split package dependencies

[mbarbin]

Added

• Added minimal library depending on sexplib0 only.

Changed

• Rewrite Fpath_base on top of Fpath_sexp0.
• Remove vendor, use expect_test_helpers_core.expect_test_helpers_base.

[coveralls]

Pull Request Test Coverage Report for Build 20

Details

• 170 of 176 (96.59%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.1%) to 98.923%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/fpath_base/src/fpath_base.ml	2	8	25.0%
<!--	Total:	170	176	96.59%	-->

Totals
Change from base Build 19:	-1.1%
Covered Lines:	551
Relevant Lines:	557

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💛 - Coveralls