Minor cleanup.

Add NewString Func (google#73)

* added utility func

* updated docs & function call

hash.go hash error covered and linter error fixed (google#71)

* hash.go hash error covered and linter error fixed

Error covered without change function definition

* Update hash.go

Reduce custom error allocation (google#70)

Zero allocation by using non-pointer error.

related google#69

name               old time/op    new time/op    delta
ParseBadLength-16    15.4ns ± 0%     3.5ns ± 0%   ~     (p=1.000 n=1+1)

name               old alloc/op   new alloc/op   delta
ParseBadLength-16     8.00B ± 0%     0.00B        ~     (p=1.000 n=1+1)

name               old allocs/op  new allocs/op  delta
ParseBadLength-16      1.00 ± 0%      0.00        ~     (p=1.000 n=1+1)

Use a custom error type for invalid lengths, replacing `fmt.Errorf` (g…

…oogle#69)

* Add benchmarks for different kinds of invalid UUIDs

Also add a test case for too-short UUIDs to ensure behavior doesn’t
change.

* Use a custom error type for invalid lengths, replacing `fmt.Errorf`

This significantly improves the speed of failed parses due to wrong
lengths. Previously the `fmt.Errorf` call dominated, making this the
most expensive error and more expensive than successfully parsing:

    BenchmarkParse-4                 29226529        36.1 ns/op
    BenchmarkParseBadLength-4         6923106       174 ns/op
    BenchmarkParseLen32Truncated-4   26641954        38.1 ns/op
    BenchmarkParseLen36Corrupted-4   19405598        59.5 ns/op

When the formatting is not required and done on-demand, the failure per
se is much faster:

    BenchmarkParse-4                 29641700        36.3 ns/op
    BenchmarkParseBadLength-4        58602537        20.0 ns/op
    BenchmarkParseLen32Truncated-4   30664791        43.6 ns/op
    BenchmarkParseLen36Corrupted-4   18882410        61.9 ns/op

Fix race in NewUUID() (google#64)

* Fixed race in NewUUID()

* Remove unnecessary variable

Fix race in NewUUID() (google#64)

* Fixed race in NewUUID()

* Remove unnecessary variable

Add darkfeline's change to set ifname if no interface is found.

Merge pull request google#38 from dmitris/go-mod

add go.mod file

Remove leftover comment.

Fix reamde to no longer say this is experimental.
The API is not going to change.