perf: replace fmt with strconv to reduce allocations

[EricGusmao]

This change replaces fmt.Sprintf with the more performant strconv package for basic type conversions.

The strconv functions are significantly faster and result in fewer memory allocations because they avoid the overhead of reflection that fmt uses for its flexibility.

[github-actions]

Benchmark Results

goos: linux
goarch: amd64
pkg: github.com/go-chi/httprate
cpu: AMD EPYC 7763 64-Core Processor                
               │ master.txt  │            pr.txt             │
               │   sec/op    │   sec/op     vs base          │
LocalCounter-4   32.21m ± 1%   32.18m ± 2%  ~ (p=0.971 n=10)

               │  master.txt  │             pr.txt             │
               │     B/op     │     B/op      vs base          │
LocalCounter-4   2.843Mi ± 0%   2.842Mi ± 0%  ~ (p=0.631 n=10)

               │ master.txt  │            pr.txt             │
               │  allocs/op  │  allocs/op   vs base          │
LocalCounter-4   121.6k ± 0%   121.5k ± 0%  ~ (p=0.646 n=10)