no_std with alloc support

[CathalMullan]

Proof of concept of how easy no_std + alloc support will be once 1.81 comes out.

Not sure we'd want to raise the msrv, however.

[codspeed-hq]

CodSpeed Performance Report

Merging #58 will degrade performances by 8.69%

_{Comparing 57-investigate-no_std-support (78a14f9) with main (a645714)}

Summary

⚡ 2 improvements
❌ 5 regressions
✅ 15 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	57-investigate-no_std-support	Change
⚡	matchit	5.2 µs	5.1 µs	+2.53%
❌	path-tree	7.8 µs	8.4 µs	-7.14%
⚡	xitca-router	7.9 µs	7.6 µs	+3.75%
❌	gonzales	48.8 µs	50.5 µs	-3.25%
❌	path-tree	51.8 µs	56.7 µs	-8.69%
❌	regex	388.8 µs	400 µs	-2.79%
❌	wayfind	44.5 µs	45.7 µs	-2.44%