add benchmarking for individual functions (#9)

* add benchmarking

* update README to include benchmarking of individual functions

* README: formatting

* README: add link to Criterion's getting_started

* shell.nix: add gnuplot

Since Criterion.rs uses gnuplot to generate nice plots, add it to the
shell.

* Cargo.toml: fixed microfetch binary name

* benchmark.rs: fix benchmark function's name

* Update README.md

---------

Co-authored-by: raf <raf@notashelf.dev>

src/lib.rs

@@ -0,0 +1,5 @@
+pub mod colors;
+pub mod desktop;
+pub mod release;
+pub mod system;
+pub mod uptime;