Optimize performance of keys definition

[Goltergaul]

This is only improving performance. Benchmark results are from this script: https://github.com/Goltergaul/definition/blob/performance/benchmark/model.rb (master vs this branch)

Before definition was slower by 1.21x, after it is faster by 1.13x in the valid data scenario

performance before:

Benchmark with valid input data:
Warming up --------------------------------------
          definition    12.167k i/100ms
          dry-struct    15.855k i/100ms
Calculating -------------------------------------
          definition    112.708k (± 5.6%) i/s -      2.251M in  20.036174s
          dry-struct    136.525k (± 2.1%) i/s -      2.743M in  20.100323s

Comparison:
          dry-struct:   136525.3 i/s
          definition:   112708.0 i/s - 1.21x  slower

Benchmark with invalid input data:
Warming up --------------------------------------
          definition     4.951k i/100ms
          dry-struct     3.166k i/100ms
Calculating -------------------------------------
          definition     50.435k (± 4.2%) i/s -      1.010M in  20.071097s
          dry-struct     32.269k (± 2.9%) i/s -    645.864k in  20.032104s

Comparison:
          definition:    50434.5 i/s
          dry-struct:    32269.2 i/s - 1.56x  slower

Performance after:

Benchmark with valid input data:
Warming up --------------------------------------
          definition    14.813k i/100ms
          dry-struct    15.516k i/100ms
Calculating -------------------------------------
          definition    150.088k (± 4.7%) i/s -      3.007M in  20.083543s
          dry-struct    132.421k (± 1.4%) i/s -      2.653M in  20.040159s

Comparison:
          definition:   150088.4 i/s
          dry-struct:   132421.5 i/s - 1.13x  slower

Benchmark with invalid input data:
Warming up --------------------------------------
          definition     6.173k i/100ms
          dry-struct     3.865k i/100ms
Calculating -------------------------------------
          definition     56.998k (± 9.4%) i/s -      1.130M in  20.046251s
          dry-struct     35.420k (±12.0%) i/s -    695.700k in  20.057381s

Comparison:
          definition:    56998.3 i/s
          dry-struct:    35420.2 i/s - 1.61x  slower