refact(validators)

[fredbi]

Readability improvements:

• Introduced private constructors for all validators
• All private constructors can use a *SchemaValidatorOptions (generalize options passing pattern)
• All exported methods use private constructors (hide the implementation of options)
• Readability: reduced the level of indentation with early exit conditions than nested conditions
• Object validator: refactored Validate in smaller functions
• Removed dead code/commented out code

Minor efficiency improvements

• In Applies method: reflect replaced by type switch whenever possible

• Removed (mostly unusable) debugLog

• In object validator:

  • pattern properties now use the regexp cache readily available
  • split path may be performed only once

• In schema props: keep arrays of pointers []*SchemaValidator and avoid copying a new schema

• In default & example validators:

  • map[string]bool -> map[string]struct{}
  • clear, don't reallocate the visited map

• []validator slice -> [n]validator array

• introduced a benchmark follow-up document, based on the existing Kubernetes API fixture

[codecov[bot]]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (ff196e5) 92.40% compared to head (486e1d2) 92.88%.

Files	Patch %	Lines
spec.go	76.66%	5 Missing and 2 partials :warning:
default_validator.go	93.18%	2 Missing and 1 partial :warning:
example_validator.go	95.65%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #169 +/- ## ========================================== + Coverage 92.40% 92.88% +0.48% ========================================== Files 22 22 Lines 2987 3277 +290 ========================================== + Hits 2760 3044 +284 - Misses 182 190 +8 + Partials 45 43 -2 ``` | [Flag](https://app.codecov.io/gh/go-openapi/validate/pull/169/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=go-openapi) | Coverage Δ | | |---|---|---| | [oldstable](https://app.codecov.io/gh/go-openapi/validate/pull/169/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=go-openapi) | `92.88% <98.65%> (+0.55%)` | :arrow_up: | | [stable](https://app.codecov.io/gh/go-openapi/validate/pull/169/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=go-openapi) | `92.88% <98.65%> (+0.48%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=go-openapi#carryforward-flags-in-the-pull-request-comment) to find out more.

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