concurrency/safe: add safe counter implementation
concurrency/safe: added mutex-free LinkedList implementation
concurrency/safe: added Len method to LinkedList
concurrency/safe: added benchmarks for LinkedList

Tested against a mutexed version. Mutexed is "old", mutex-free is "new":

benchmark            old ns/op    new ns/op    delta
BenchmarkInsert            310          190  -38,71%
BenchmarkInsert-2          313          204  -34,82%
BenchmarkInsert-4          344          222  -35,47%
BenchmarkInsert-8          338          217  -35,80%
BenchmarkRemove            231          121  -47,62%
BenchmarkRemove-2          239          126  -47,28%
BenchmarkRemove-4          235          123  -47,66%
BenchmarkRemove-8          226          121  -46,46%

benchmark           old allocs   new allocs    delta
BenchmarkInsert              1            1    0,00%
BenchmarkInsert-2            1            1    0,00%
BenchmarkInsert-4            1            1    0,00%
BenchmarkInsert-8            1            1    0,00%
BenchmarkRemove              0            0     n/a%
BenchmarkRemove-2            0            0     n/a%
BenchmarkRemove-4            0            0     n/a%
BenchmarkRemove-8            0            0     n/a%

benchmark            old bytes    new bytes    delta
BenchmarkInsert             17           17    0,00%
BenchmarkInsert-2           17           17    0,00%
BenchmarkInsert-4           17           17    0,00%
BenchmarkInsert-8           17           17    0,00%
BenchmarkRemove             16           17    6,25%
BenchmarkRemove-2           16           17    6,25%
BenchmarkRemove-4           16           17    6,25%
BenchmarkRemove-8           15           17   13,33%