address some linting issues, and small refactor, performance optimizations

[thaJeztah]

address some linting issues

• rename "digester" vars that shadowed package-level digester type
• rename "hexdigestbytes" to be properly camelCase
• update test to use "errors.Is()" instead of straight compare

testdigest: fix package GoDoc to be in the right format

remove import aliases

RegisterAlgorithm: skip regex if not needed

No need to validate the format if we previously accepted it to be
registered.

use errors.New() for errors that don't need formatting

digestset: report allocations in benchmarks

Just for convenience ':-)

Before:

pkg: github.com/opencontainers/go-digest/digestset
BenchmarkAdd10-10              183613      6368 ns/op
BenchmarkAdd100-10              15739     75741 ns/op
BenchmarkAdd1000-10              1430    831884 ns/op
BenchmarkRemove10-10           169999      6984 ns/op
BenchmarkRemove100-10           15312     78299 ns/op
BenchmarkRemove1000-10           1470    815144 ns/op
BenchmarkLookup10-10         27067897        43.69 ns/op
BenchmarkLookup100-10        17203653        69.17 ns/op
BenchmarkLookup1000-10       13043708        96.32 ns/op
BenchmarkShortCode10-10       3574094       334.5 ns/op
BenchmarkShortCode100-10       320438      3782 ns/op
BenchmarkShortCode1000-10       29110     41460 ns/op

After:

pkg: github.com/opencontainers/go-digest/digestset
BenchmarkAdd10
BenchmarkAdd10-10              176776      6400 ns/op         608 B/op      12 allocs/op
BenchmarkAdd100-10              15267     76016 ns/op        5746 B/op     102 allocs/op
BenchmarkAdd1000-10              1426    833977 ns/op       56267 B/op    1002 allocs/op
BenchmarkRemove10-10           169448      7043 ns/op         128 B/op       2 allocs/op
BenchmarkRemove100-10           15249     78730 ns/op         944 B/op       2 allocs/op
BenchmarkRemove1000-10           1444    819247 ns/op        8240 B/op       2 allocs/op
BenchmarkLookup10-10         27042379        45.68 ns/op        0 B/op       0 allocs/op
BenchmarkLookup100-10        17098725        69.72 ns/op        0 B/op       0 allocs/op
BenchmarkLookup1000-10       12769602        92.97 ns/op        0 B/op       0 allocs/op
BenchmarkShortCode10-10       3551202       332.1 ns/op	      630 B/op       2 allocs/op
BenchmarkShortCode100-10       313929      3805 ns/op        5449 B/op       4 allocs/op
BenchmarkShortCode1000-10       28963     41373 ns/op       81992 B/op       3 allocs/op

remove unneeded fmt.Sprintf and indirections

Remove fmt.Sprintf and use string-concatenation instead to reduce
some allocations.

Before / After:

BenchmarkNewDigestFromEncoded-10     8474174   128.4 ns/op   112 B/op  3 allocs/op
BenchmarkNewDigestFromEncoded-10    37912695    31.55 ns/op   80 B/op  1 allocs/op

BenchmarkNewDigestFromBytes-10       5087299    237.2 ns/op	 200 B/op  5 allocs/op
BenchmarkNewDigestFromBytes-10       8416543    146.8 ns/op	 168 B/op  3 allocs/op