Pretty printer

[ericzhang6222]

Fixes #171 .

Changes proposed in this pull request:

• Add a pretty print function to arrai itself. //fmt.pretty(value) which can print more readable format of string represents rel.Value:

  {
  b: 2,
  a: 1,
  c: (
  d: 11,
  f: {
    111,
    222
  },
  e: 22
  )
  }

Checklist:

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

[coveralls]

Pull Request Test Coverage Report for Build 734ecd1e2bdb35a0b6104d6488ad4100a0707885-PR-511

• 83 of 98 (84.69%) changed or added relevant lines in 2 files are covered.
• 9 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.3%) to 47.776%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
syntax/std_fmt.go	8	10	80.0%
syntax/format.go	75	88	85.23%
<!--	Total:	83	98	84.69%	-->

Files with Coverage Reduction	New Missed Lines	%
syntax/import_cache.go	1	77.27%
rel/value_set_generic.go	2	74.47%
rel/value_set_str.go	6	60.23%
<!--	Total:	9		-->

Totals
Change from base Build e48178762c57f67f5ccfd3c330783a6fcfb5c785:	0.3%
Covered Lines:	4382
Relevant Lines:	9172

---

💛 - Coveralls

[marcelocantos]

(In a separate PR) relations should be formatted as tables, e.g.: for a "simple" table: {|a, b| (1, 2), (12, 13)}:

{|a , b |
 (1 , 2 ),
 (13, 14),
}

For a "complex" table, revert to set-of-tuples representation:

{
  (
    a: {1, 2},
    b: {3, 4},
  ),
  (
    a: {20, 30},
    b: {40, 50},
  ),
}

[ericzhang6222]

Simple structures should be inlined. E.g., [1, 2, 3] should format as is.

There's no hard and fast rule for what constitutes "simple". As a bare minimum, inline any array or set (up to some threshold length) with only trivial elements (numbers, strings, byte arrays, (), {} and true).

The threshold could be fixed, or maybe a function of the current indent level.

Will do it in a separated PR. As @nofun97 want a MVP to help debugging ASAP, so I didn't do in this PR.