Implement `//arrai.info` to print arrai build information

[ericzhang6222]

Fixes #522

Changes proposed in this pull request:

• Implements //arrai.info to print arr.ai build information

Current output is:

@> //arrai.info
(date: '2020-07-24T08:09:49Z', git: (commit: '52c577427456bee1754840cbe53455c0809a1e92', tags: {'v0.101.0'}), go: (arch: 'amd64', compiler: (arch: 'amd64', os: 'darwin', version: 'go1.14'), os: 'darwin'), version: 'DIRTY-52c577427456bee1754840cbe53455c0809a1e92 (from v0.101.0)')
@>  

And prettified output looks like:

@> //arrai.info
(
  date: '2020-07-24T08:09:49Z', 
  git: (
    commit: '52c577427456bee1754840cbe53455c0809a1e92', 
    tags: {'v0.101.0'}
  ), 
  go: (
    arch: 'amd64', 
    compiler: (
      arch: 'amd64', 
      os: 'darwin', 
      version: 'go1.14'
    ), 
    os: 'darwin'
  ), 
  version: 'DIRTY-52c577427456bee1754840cbe53455c0809a1e92 (from v0.101.0)'
)
@>  

Pretty output will be done by https://github.com/arr-ai/arrai/issues/171#issuecomment-662364599

Checklist:

• [x] Added related tests
• [] Made corresponding changes to the documentation

[coveralls]

Pull Request Test Coverage Report for Build 1dd1b2a2657dff969e1942c00b4d4dde034a005c-PR-521

• 25 of 28 (89.29%) changed or added relevant lines in 3 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 48.067%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/arrai/info.go	0	1	0.0%
cmd/arrai/main.go	0	2	0.0%
<!--	Total:	25	28	89.29%	-->

Files with Coverage Reduction	New Missed Lines	%
rel/value_set_generic.go	2	74.47%
<!--	Total:	2		-->

Totals
Change from base Build 4b96030891b5d47fba32db56c3aa170400fe69d9:	0.2%
Covered Lines:	4426
Relevant Lines:	9208

---

💛 - Coveralls