Feature gate multi-threaded sorting

[nessex]

WHAT

Feature gate multi-threaded sorting, behind a default feature.

This allows you to remove all multi-threading related logic by using:
default-features = false

WHY

Multi-threaded sorting requires importing rayon. If you are only using this library in a single-threaded configuration, this introduces a lot of unnecessary overhead and bloat.

Fixes: #2

[nessex]

Test results

Before

$ cargo bloat --release --crates
    Finished release [optimized] target(s) in 0.00s
    Analyzing target/release/rdst-bloat-test

 File  .text     Size Crate
37.4%  73.1% 275.2KiB std
 6.9%  13.5%  50.9KiB rayon_core
 2.9%   5.6%  21.3KiB rdst
 1.8%   3.5%  13.3KiB rayon
 1.3%   2.5%   9.3KiB crossbeam_epoch
 0.7%   1.4%   5.3KiB [Unknown]
 0.6%   1.3%   4.7KiB crossbeam_channel
 0.5%   0.9%   3.4KiB crossbeam_deque
 0.0%   0.1%     244B rdst_bloat_test
 0.0%   0.0%      32B num_cpus
51.2% 100.0% 376.6KiB .text section size, the file size is 735.8KiB

Note: numbers above are a result of guesswork. They are not 100% correct and never will be.

After

Using default-features = false

$ cargo bloat --release --crates
   Compiling rdst v0.19.2 (/Users/n/dev/github.com/Nessex/rdst)
   Compiling rdst-bloat-test v0.1.0 (/Users/n/dev/github.com/Nessex/rdst-bloat-test)
    Finished release [optimized] target(s) in 0.42s
    Analyzing target/release/rdst-bloat-test

 File  .text     Size Crate
46.5%  94.9% 199.5KiB std
 2.8%   5.7%  11.9KiB rdst
 0.5%   1.1%   2.4KiB [Unknown]
 0.1%   0.1%     240B rdst_bloat_test
49.0% 100.0% 210.2KiB .text section size, the file size is 429.4KiB

Note: numbers above are a result of guesswork. They are not 100% correct and never will be.