v2.1.0: ApplyPtr and DiffPtr

This release adds ApplyPtr and DiffPtr methods to Update[T] via #12.

v2.0.0: Generic Update Types

Version 2 Features

Version 2 replaces the Nullable interface and various implementations
(including user-defined implementations!) with just two generic types:
Update[T] and SliceUpdate[T].

Update uses a value internally, rather than a pointer. This may reduce the
need for allocations, and it also means that updates can now be meaningfully
compared using ==.

In nullable version 1, only nullable.String implements fmt.Stringer. With
go 1.18 generics, it's not possible to implement an interface only for
particular type parameters of a generic type, leaving us with the choice to
implement fmt.Stringer for all types or for none. We chose the former. The
Update implementation uses a type switch to check if T is itself a string
or fmt.Stringer, falling back to fmt.Sprintf with the %v verb for all
other types. (The SliceUpdate[T] implementation always uses fmt.Sprintf with
%v since a []T is not string or fmt.Stringer.)

Migration Guide

The package has been renamed from nullable to nup (for "nullable update").
It's 62.5% shorter and 87% more whimsical! You should replace v1 imports
(github.com/nicheinc/nullable) with github.com/nicheinc/nullable/v2/nup.

The old Value method on each Nullable type has been replaced with three
Update/SliceUpdatge methods: Value, ValueOperation, and ValueOrNil:

• The new Value method returns two values: value and isSet. isSet
  indicates whether the update is a set operation. If isSet is true, then
  value is the update's set value. If isSet is false, then value is the
  zero value for the update's type parameter.
• ValueOperation is similar to the new Value method but returns value and
  operation. value is the zero value unless operation == OpSet. This
  method is useful for handling all three update types in one switch statement,
  for example.
• ValueOrNil behaves similarly to the old Value methods, returning a value
  pointer that is non-nil only if the update is a set operation. However,
  unlike the old methods, the returned pointer references a fresh copy of the
  contained value, so it can't be used to mutate the update.

The following table illustrates some of the other noteworthy differences between
version 1 and 2:

Version 1	Version 2
nullable.Int	nup.Update[int]
nullable.NewIntPtr(p)	nup.RemoveOrSet(p)
u.IsSet()	u.IsChange()
u.Removed()	u.IsRemove()
u.Equals(5)	u.IsSetTo(5)
u.IsNegative()	u.IsSetSuchThat(func(v int) bool { return v < 0 })

The semantics of IsSet() have changed from the version 1 types. Previously,
IsSet() returned true if the update was a set or removal update. In version
2, a given update is exactly one of three operations: no-op, remove, or set.
Thus IsSet() is now mutually exclusive to IsRemove (Removed(), in version
1). Suppose you had a variable update of type nullable.Int, and you used
update.IsSet() to check that update is not the zero value. Then you should
not only change update's type to nup.Update[int] because IsSet() will no
longer return true if update is a remove operation. You should instead use
update.IsChange().

The following update-type-specific value checks have been removed: Int.IsZero,
Int.IsNegative, Float64.IsZero, Float64.IsNegative, String.IsEmpty, and
StringSlice.IsEmpty. If go generics are ever extended to support type
constraints on generic methods, we could restore some of these methods. For now,
use IsSetTo or IsSetSuchThat with a suitable argument (as shown in the table
above). Another option is to implement the removed type-specific methods you
care about as free functions, for instance:

func IsNegative(update nup.Update[int]) bool {
	value, ok := update.Value()
	return ok && value < 0
}

Clean up golint warnings and remove coverage badge

Merge pull request #8 from nicheinc/enhancement/golint-cleanup

Clean up golint warnings and remove coverage badge

Open-source under the MIT license

Merge pull request #7 from nicheinc/feature/open-source

Open-source the library

v1.3.0 Equals, Apply, Apply, Diff

Add Equals, Apply, ApplyPtr, and Diff methods to each type.

Export the InterfaceValue method

Merge pull request #3 from nicheinc/bug/i2

Export the InterfaceValue method

Stringy Strings

nullable.String now implements Stringer.

Version 1.0.0

This package has now been QA'd via its use in both entity and search so should be ready for v1.0.0.

v0.1.0

Support marshalling non-structs

v0.0.0

Remove partial doc string