Add -validate flag generating a Validate() error method

[dmarkham]

Based on #102 by @peczenyj, itself a port of #82 by @mbyio. Rebuilt on current master.

-validate generates:

func (i Color) Validate() error {
	if !i.IsAColor() {
		return fmt.Errorf("%v does not belong to Color values", i)
	}
	return nil
}

With -typederrors the error wraps enumerrs.ErrValueInvalid, same as TString() and the unmarshal methods. Opt-in, stdlib only, no change to existing output.

Tests

• testdata/validate.go: contiguous enum. Every declared value returns nil; below, just above, and far out of range each return an error with the exact message; calls through a Validate() error interface.
• testdata/validateTypedErrors.go: enum with gaps. Declared values return nil and do not match the sentinel; gap, zero, negative and out-of-range values match enumerrs.ErrValueInvalid under errors.Is and name the value; an unrelated sentinel does not match.
• Two new goldens, plain and typed. No existing golden changed.
• Mutation check: a Validate() that always returns nil and a builder that ignores -typederrors each fail the suite.

The end-to-end harness now takes extra flags per fixture instead of one typed-errors boolean.