crud: add readview support

[better0fdead]

Added readview support for select and pairs.
e.g

tarantool> a = crud.readview('1')

tarantool> crud.select('customers', nil, {fullscan=true})
---
- metadata: [{'name': 'id', 'type': 'unsigned'}, {'name': 'bucket_id', 'type': 'unsigned'},
    {'name': 'name', 'type': 'string'}, {'name': 'age', 'type': 'number'}]
  rows:
  - [1, 477, 'Elizabeth', 12]
  - [2, 401, 'Mary', 46]
  - [3, 2804, 'David', 33]
  - [4, 1161, 'William', 81]
  - [5, 1172, 'Jack', 35]
  - [6, 1064, 'William', 25]
  - [7, 693, 'Elizabeth', 18]
- null
...

tarantool> a:select('customers', nil, {batch_size=1,fullscan=true})
---
- metadata: [{'name': 'id', 'type': 'unsigned'}, {'name': 'bucket_id', 'type': 'unsigned'},
    {'name': 'name', 'type': 'string'}, {'name': 'age', 'type': 'number'}]
  rows:
  - [1, 477, 'Elizabeth', 12]
  - [2, 401, 'Mary', 46]
  - [3, 2804, 'David', 33]
  - [4, 1161, 'William', 81]
  - [5, 1172, 'Jack', 35]
  - [6, 1064, 'William', 25]
  - [7, 693, 'Elizabeth', 18]
- null
...

tarantool> crud.insert('customers', {8, box.NULL, 'Elizabeth', 23})
---
- rows:
  - [8, 185, 'Elizabeth', 23]
  metadata: [{'name': 'id', 'type': 'unsigned'}, {'name': 'bucket_id', 'type': 'unsigned'},
    {'name': 'name', 'type': 'string'}, {'name': 'age', 'type': 'number'}]
- null
...

tarantool> a:select('customers', nil, {batch_size=1,fullscan=true})
---
- metadata: [{'name': 'id', 'type': 'unsigned'}, {'name': 'bucket_id', 'type': 'unsigned'},
    {'name': 'name', 'type': 'string'}, {'name': 'age', 'type': 'number'}]
  rows:
  - [1, 477, 'Elizabeth', 12]
  - [2, 401, 'Mary', 46]
  - [3, 2804, 'David', 33]
  - [4, 1161, 'William', 81]
  - [5, 1172, 'Jack', 35]
  - [6, 1064, 'William', 25]
  - [7, 693, 'Elizabeth', 18]
- null
...

tarantool> crud.select('customers', nil, {fullscan=true})
---
- metadata: [{'name': 'id', 'type': 'unsigned'}, {'name': 'bucket_id', 'type': 'unsigned'},
    {'name': 'name', 'type': 'string'}, {'name': 'age', 'type': 'number'}]
  rows:
  - [1, 477, 'Elizabeth', 12]
  - [2, 401, 'Mary', 46]
  - [3, 2804, 'David', 33]
  - [4, 1161, 'William', 81]
  - [5, 1172, 'Jack', 35]
  - [6, 1064, 'William', 25]
  - [7, 693, 'Elizabeth', 18]
  - [8, 185, 'Elizabeth', 23]
- null
...
tarantool> tuples = {}
---
...

tarantool> for _, tuple in a:pairs('customers', {{'<=', 'age', 35}},{use_tomap=false})  do
table.insert(tuples, tuple)
end
---
...

tarantool> tuples
---
- - [5, 1172, 'Jack', 35]
  - [3, 2804, 'David', 33]
  - [6, 1064, 'William', 25]
  - [7, 693, 'Elizabeth', 18]
  - [1, 477, 'Elizabeth', 12]
...

Closes #343

I didn't forget about

• [ x] Tests
• [ x] Changelog
• [ x] Documentation

[DifferentialOrange]

Thank you for your patch! That's a rather nice implementation, but we may improve it in some ways. A couple of things definitely need to be changed, some of them may be ignored for now.

[DifferentialOrange]

Thank you for the updates. Most of the crucial things seems fine, there are some minor things that should be fixed before merge. And we still lack some tests. You may start to write README API description after adding missing options to .new and :close

[DifferentialOrange]

Thank you for the updates! Seems awesome, I think I'll merge this one after you resolve remaining comments.