1. libct/system/proc_test: fix, improve, add benchmarks

1. Add a test case that tests parentheses in command.

2. Replace individual comparisons with reflect.DeepEqual.
   This also fixes wrong %-style types used in Fatalf statements.

3. Replace Fatalf with Errorf so we don't bail out on the first
   failure, and do not check result on error.

4. Add two benchmarks.

2. libct/system.Stat: fix/improve/speedup

1. Remove PID field. It is totally useless -- we already know the PID.

2. Rewrite parseStat() to make it faster and more correct:

   • do not use fmt.Scanf as it is very slow;
   • avoid splitting data into 20+ fields, of which we only need 2 (should also help garbage collector);
   • make sure to not panic on short lines and other bad input;
   • add some bad input tests (some fail with old code);
   • use LastIndexByte instead of LastIndex where appropriate.

Benchmarks:

before (from the previous commit message):

BenchmarkParseStat-4              116415             10804 ns/op
BenchmarkParseRealStat-4             240           4781769 ns/op

after:

BenchmarkParseStat-4           1164948              1068 ns/op
BenchmarkParseRealStat-4           331           3458315 ns/op

We are seeing 10x speedup in a synthetic benchmark, and about 1.4x
speedup in a real world benchmark. All this while being more scrupulous
about any possible errors. I also suspect much less allocations and thus
garbage to collect.

While at it, do not ignore any possible errors, and properly wrap those.

3. Add I and P process states (available since Linux 4.14).

History

• v2: use pkg/errors more, remove t.Logf from test
• v3: rebased; drop pkg/errors; gofumpt'ed
• v4: rebased; improved description
• v5: rebased; mention bad input tests, added second benchmark results
• v6: remove PID field, do not use strings.Split, further speedup.
• v7: readability improvements, added new process states.
• v8: improved parser to bail earier on incorrect data, more doc, more tests.