Added NormalizedString to be used as key in maps

[cmaglie]

Sometimes it's useful to have a map like map[string]*Release that maps a version to the corresponding release. Anyway, this has a drawback, if the version is not normalized before being assigned as a key we may end up having something like:

"1.0": &Release{...},
"1.0.0": &Release{...},

where two different entries are present for the same version.
To avoid this kind of problem this PR adds a new type NormalizedString, so the previous map can be rewritten as map[semver.NormalizedString]*Release. To obtain the normalized string the method Version.NormalizedString() can be used.

releases := map[semver.NormalizedString]*Release{}

v10 := semver.MustParse("1.0")
releases[v10.NormalizedString()] = release1

v100 := semver.MustParse("1.0.0")
releases[v100.NormalizedString()] = release2  // replaces "release1"

r := releases[v10.NormalizedString()] // gets "release2"
fmt.Println(v10.Equal(v100)) // prints "true"

it also helps with a compile-time sanity check on code doing the wrong thing:

releases[v10.String()] = release1
// error: cannot use v10.String() (value of type string) as semver.NormalizedString value in map

[codecov]

Codecov Report

Merging #12 (9dd7f57) into master (518e126) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #12   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          761       804   +43     
=========================================
+ Hits           761       804   +43     

Impacted Files	Coverage Δ
relaxed_version.go	100.00% <100.00%> (ø)
version.go	100.00% <100.00%> (ø)

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