Add methods to IPRange for parity with other types

[Joe Tsai]

The IPRange lack some functions and methods that consistently exist onIP, IPPort, and IPPrefix. See the following table.

func	IP	IPPort	IPPrefix	IPRange
func ParseT(string) (T, error)	✅	✅	✅	✅
func MustParseT(string) T	✅	✅	✅	❌
func (T) IsValid() bool	✅	✅	✅	✅
func (T) IsZero() bool	✅	✅	✅	❌
func (T) String() string	✅	✅	✅	✅
func (T) AppendTo([]byte) []byte	✅	✅	✅	❌
func (T) MarshalText() ([]byte, error)	✅	✅	✅	❌
func (*T) UnmarshalText([]byte) error	✅	✅	✅	❌

Given that a parse method exists already for IPRange, I don't see any reason why we can't also provide MustParse, AppendTo, MarshalText, and UnmarshalText.

The use case of this change is for tailscale.com/util/deephash, where the netaddr.IPRange type is the only type ever visited by deephash where the package must touch unexported fields because the type has no AppendTo method (which deephash specially understands). Adding AppendTo would be a one step closer to generating deephash functionality for all relevant tailscale types.

\cc @bradfitz @josharian

[Brad Fitzpatrick]

SGTM

[Josh Bleecher Snyder]

Make it so!