Add `ulid.Zero` and `.IsZero()` method (#112)

* Add `uuid.Nil` and `.IsZero()` method

This simplifies comparisons with empty ULIDs.  Right now, most people
are doing something like the following to check if a ULID is non-zero:

```go
id.Compare(ulid.ULID{}) == 0
```

This requires allocating a new empty ULID each time.  Some packages
avoid this by creating their own global (or private) nil ULIDs on
initialization.

This should be built in and simple for better perf & a cleaner API.

* Update ulid.go

* Update ulid.go

* Update ulid.go

* Update ulid_test.go

* Update ulid_test.go

---------

Co-authored-by: Peter Bourgon <[email protected]>

ulid: add DefaultEntropy() and Make() (#81)

Users of this library have opened many issues regarding the difficulty
of choosing an entropy source that is safe for concurrent use.

This commit introduces a thread safe per process monotonically increase `DefaultEntropy()`
function as well as an easy to use `Make()` function, aimed at users
that want safe defaults chosen for them.

Co-authored-by: Peter Bourgon <[email protected]>

ulid: add DefaultEntropy() and Make() (#81)

Users of this library have opened many issues regarding the difficulty
of choosing an entropy source that is safe for concurrent use.

This commit introduces a thread safe per process monotonically increase `DefaultEntropy()`
function as well as an easy to use `Make()` function, aimed at users
that want safe defaults chosen for them.

Co-authored-by: Peter Bourgon <[email protected]>

Add /v2 suffix to import (#47)

If we opt-in to modules with a non-v0 or v1 version, we have to
explicitly append /vX to the import path whenever we import.

This is infuriating.

go.mod: put v2 in the module path

TestMonotonicSafe fails sometimes (#44)

* TestMonotonicSafe fails sometimes

* TestMonotonicSafe: vary seed and inc

* Check for MonotonicReader interface instead of *monotonic

* rm seed/inc variation in test, it's not relevant

* TestMonotonicSafe should use multiple goroutines

* t.Fatalf must be called in the same goroutine as the test

Release 1.3.1

Release 1.3.0

Release 1.2.0

Release 1.1.0