use standalone purrr

[JosiahParry]

No description provided.

[JosiahParry]

This PR removes {purrr} as a dependency by using the standalone purrr from https://github.com/r-lib/rlang/blob/main/R/standalone-purrr.R

This doesn't have full coverage of purrr functionality but I believe I've handled all the edge cases.

[Ilia-Kosenkov]

Dropping a dependency is worth mentioning on the NEWS.md.

[JosiahParry]

<3 thank you!

[codecov]

Codecov Report

Attention: Patch coverage is 97.29730% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.10%. Comparing base (b20d88c) to head (8632d20).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
R/toml_serialization.R	87.50%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
R/create_extendr_package.R	100.00% <100.00%> (ø)
R/eval.R	100.00% <100.00%> (ø)
R/find_exports.R	100.00% <100.00%> (ø)
R/function_options.R	92.18% <100.00%> (ø)
R/generate_toml.R	100.00% <100.00%> (ø)
R/sanitize_code.R	98.90% <100.00%> (-0.05%)	⬇️
R/source.R	76.41% <100.00%> (ø)
R/track_rust_source.R	97.36% <100.00%> (ø)
R/toml_serialization.R	82.05% <87.50%> (ø)