Use MapRange() to iterate maps

[bep]

       │ master.bench │         fix-mapkeys.bench          │
       │    sec/op    │   sec/op     vs base               │
Map-10    3.598µ ± 1%   3.058µ ± 0%  -15.00% (p=0.002 n=6)

       │ master.bench │          fix-mapkeys.bench          │
       │     B/op     │     B/op      vs base               │
Map-10   1.844Ki ± 0%   1.328Ki ± 0%  -27.97% (p=0.002 n=6)

       │ master.bench │        fix-mapkeys.bench         │
       │  allocs/op   │ allocs/op   vs base              │
Map-10     130.0 ± 0%   128.0 ± 0%  -1.54% (p=0.002 n=6)