fix: panics when private field is validated

[ykalchevskiy]

Fixes Or Enhances

dive, omitnil and time-related validations panic by themselves.
panic inside panic happens for most of the checks in case of bad type.

(reflect.Value).Interface() can't be used if a field is private. getValue must be used instead.
In case of casting to string, fmt is smart enough by itself.
In case of getting type with %T, (reflect.Value).Type() can be safely used.

Without these changes tests fail with this error:

panic: reflect.Value.Interface: cannot return value obtained from unexported field or method [recovered]
        panic: reflect.Value.Interface: cannot return value obtained from unexported field or method

goroutine 22 [running]:
testing.tRunner.func1.2({0x10137e740, 0x101416580})
        /Users/yan/.go/src/testing/testing.go:1734 +0x1ac
testing.tRunner.func1()
        /Users/yan/.go/src/testing/testing.go:1737 +0x334
panic({0x10137e740?, 0x101416580?})
        /Users/yan/.go/src/runtime/panic.go:792 +0x124
reflect.valueInterface({0x10137e740?, 0x140002183a0?, 0x10141c850?}, 0xa0?)
        /Users/yan/.go/src/reflect/value.go:1495 +0xc8
reflect.Value.Interface(...)
        /Users/yan/.go/src/reflect/value.go:1484
github.com/go-playground/validator/v10.(*validate).traverseField(0x140001805a0, {0x10141a610, 0x1016cd0a0}, {0x1013ba060?, 0x1400008f550?, 0x17?}, {0x1013a7500?, 0x1400008f550?, 0x10137e980?}, {0x14000190340, ...}, ...)
        /Users/yan/projects/validator/validator.go:315 +0x222c
github.com/go-playground/validator/v10.(*validate).validateStruct(0x140001805a0, {0x10141a610, 0x1016cd0a0}, {0x101377bc0?, 0x1400008f550?, 0x101?}, {0x1013ba060?, 0x1400008f550?, 0x10137e980?}, {0x10141fd28, ...}, ...)
        /Users/yan/projects/validator/validator.go:72 +0x50c
github.com/go-playground/validator/v10.(*Validate).StructCtx(0x140000d03f0, {0x10141a610, 0x1016cd0a0}, {0x101377bc0, 0x1400008f550})
        /Users/yan/projects/validator/validator_instance.go:372 +0x324
github.com/go-playground/validator/v10.(*Validate).Struct(...)
        /Users/yan/projects/validator/validator_instance.go:346
github.com/go-playground/validator/v10.TestPrivateFieldsStruct(0x14000082fc0)
        /Users/yan/projects/validator/validator_test.go:14181 +0x368
testing.tRunner(0x14000082fc0, 0x101413400)
        /Users/yan/.go/src/testing/testing.go:1792 +0xe4
created by testing.(*T).Run in goroutine 1
        /Users/yan/.go/src/testing/testing.go:1851 +0x374
FAIL    github.com/go-playground/validator/v10  1.197s

Make sure that you've checked the boxes below before you submit PR:

• Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[coveralls]

coverage: 73.662% (+0.007%) from 73.655%
when pulling afc7459 on ykalchevskiy:master
into 5b31512 on go-playground:master.