Increase performance of nested keys search

[benoitmasson]

This follows PR #253, which closed Issue #249 induced by PR #195.

It generalizes to nested keys the improvement from PR #253 which consists in assuming that all keys are lower-cased, thus allowing to search for values by writing

val = m[key]

instead of

for k, v := range m {
  if strings.ToLower(k) == strings.ToLower(key) {
    val = v
  }
}

This induces a huge performance gain.

Function insensitiviseMap() was also fixed (it did not recurse correctly through the inner maps).

@bep my performance tests show no performance loss (and even very little gain) from your version, can I let you benchmark on your hardware to make sure I didn't remove anything essential (it should be fine, but let's double-check this time...)?

If that's OK for you, I think this would be a more generic solution than your patch, less likely to be bypassed in the future.

[bep]

This looks very good in general, but it is slower and more memory hungry for the common case...

▶ benchcmp /tmp/1.bench /tmp/2.bench
benchmark                     old ns/op     new ns/op     delta
BenchmarkGetBool-4            553           562           +1.63%
BenchmarkGet-4                452           477           +5.53%
BenchmarkGetBoolFromMap-4     7.00          6.51          -7.00%

benchmark                     old allocs     new allocs     delta
BenchmarkGetBool-4            4              5              +25.00%
BenchmarkGet-4                4              5              +25.00%
BenchmarkGetBoolFromMap-4     0              0              +0.00%

benchmark                     old bytes     new bytes     delta
BenchmarkGetBool-4            49            65            +32.65%
BenchmarkGet-4                48            64            +33.33%
BenchmarkGetBoolFromMap-4     0             0             +0.00%

So, if you reintroduce my searchMapForKey method the above be fine.