feat(validate): initial implementation

This is a bare-bones implementation of a basic validation package based
around a very simply Validatable interface:

    type Validatable interface {
        Validate() error
    }

The goal is to keep things as simple as possible, while also giving as
much control as possible over how validation logic is performed.

error.go

@@ -0,0 +1,67 @@
+package validate
+
+import (
+	"errors"
+	"fmt"
+
+	"go.uber.org/multierr"
+)
+
+// Error represents validation errors, and implements Go's error type. Field
+// indicates the struct field the validation error is relevant to, which is the
+// full nested path relative to the top-level object being validated.
+type Error struct {
+	Field string
+	Msg   string
+	Err   error
+}
+
+func (s *Error) Error() string {
+	msg := s.Msg
+	if msg == "" && s.Err != nil {
+		msg = s.Err.Error()
+	}
+
+	if msg == "" {
+		msg = "unknown error"
+	}
+
+	if s.Field == "" {
+		return msg
+	}
+
+	return fmt.Sprintf("%s: %s", s.Field, msg)
+}
+
+func (s *Error) Is(target error) bool {
+	return errors.Is(s.Err, target)
+}
+
+func (s *Error) Unwrap() error {
+	return s.Err
+}
+
+// Append combines two errors together into a single new error which internally
+// keeps track of multiple errors via go.uber.org/multierr. If either error is a
+// previously combined multierr, the returned error will be a flattened list of
+// all errors.
+func Append(errs error, err error) error {
+	return multierr.Append(errs, err)
+}
+
+// AppendError appends a new *Error type to errs with the Msg field populated
+// with the provided msg.
+func AppendError(errs error, msg string) error {
+	return multierr.Append(errs, &Error{Msg: msg})
+}
+
+// AppendFieldError appends a new *Error type to errs with Field and Msg
+// populated with given field and msg values.
+func AppendFieldError(errs error, field, msg string) error {
+	return multierr.Append(errs, &Error{Field: field, Msg: msg})
+}
+
+// Errors returns a slice of all errors appended into the given error.
+func Errors(err error) []error {
+	return multierr.Errors(err)
+}