Closes #4333

@TarantoolBot document
Title: Document decimal field type.

Decimals may now be stored in spaces. A corresponding field type is
introduced: 'decimal'. Decimal values are also allowed in 'scalar',
'any' and 'number' fields.

'decimal' field type is appropriate for both memtx HASH and TREE
indices, as well as for vinyl TREE index.

```
To create an index 'pk' over a decimal field, say
```
tarantool> box.space.test:create_index('pk', {parts={1, 'decimal'}})
---
- unique: true
  parts:
  - type: decimal
    is_nullable: false
    fieldno: 1
  id: 0
  space_id: 512
  type: TREE
  name: pk
...
```
Now you can insert some decimal values:
```
tarantool> for i = 1,10 do
         > box.space.test:insert{decimal.new((i-5)/10)}
         > end
---
...
```
tarantool> box.space.test:select{}
---
- - [-0.4]
  - [-0.3]
  - [-0.2]
  - [-0.1]
  - [0]
  - [0.1]
  - [0.2]
  - [0.3]
  - [0.4]
  - [0.5]
...
```
Decimals may alse be inserted into `scalar` and `number` fields. In this
case all the number values are sorted correctly:
```
tarantool> box.schema.space.create('test')
tarantool> box.space.test:create_index('pk', {parts={1, 'number'}})
tarantool> box.space.test:insert{-1.0001, 'number'}
---
- [-1.0001, 'number']
...

tarantool> box.space.test:insert{decimal.new(-1.00001), 'decimal'}
---
- [-1.00001, 'decimal']
...

tarantool> box.space.test:insert{-1, 'number'}
---
- [-1, 'number']
...

tarantool> box.space.test:insert{decimal.new(-0.999), 'decimal'}
---
- [-0.999, 'decimal']
...

tarantool> box.space.test:insert{-0.998, 'number'}
---
- [-0.998, 'number']
...

tarantool> box.space.test:insert{-0.9, 'number'}
---
- [-0.9, 'number']
...

tarantool> box.space.test:insert{-0.95, 'number'}
---
- [-0.95, 'number']
...

tarantool> box.space.test:insert{decimal.new(-0.92), 'decimal'}
---
- [-0.92, 'decimal']
...

tarantool> box.space.test:insert{decimal.new(-0.971), 'decimal'}
---
- [-0.971, 'decimal']
...

tarantool> box.space.test:select{}
---
- - [-1.0001, 'number']
  - [-1.00001, 'decimal']
  - [-1, 'number']
  - [-0.999, 'decimal']
  - [-0.998, 'number']
  - [-0.971, 'decimal']
  - [-0.95, 'number']
  - [-0.92, 'decimal']
  - [-0.9, 'number']
...

```
Uniqueness is also preserved between decimals and other number types:
```
tarantool> box.space.test:insert{-0.92}
---
- error: Duplicate key exists in unique index 'pk' in space 'test'
...

tarantool> box.space.test:insert{decimal.new(-0.9)}
---
- error: Duplicate key exists in unique index 'pk' in space 'test'
...

```

You can also set decimal fields in space format:
```
tarantool> _ = box.schema.space.create('test')
---
...

tarantool> _ = box.space.test:create_index('pk')
---
...

tarantool> box.space.test:format{{name='id', type='unsigned'}, {name='balance', type='decimal'}}
---
...

tarantool> box.space.test:insert{1}
---
- error: Tuple field 2 required by space format is missing
...

tarantool> box.space.test:insert{1, 'string'}
---
- error: 'Tuple field 2 type does not match one required by operation: expected decimal'
...

tarantool> box.space.test:insert{1, 1.2345}
---
- error: 'Tuple field 2 type does not match one required by operation: expected decimal'
...

tarantool> box.space.test:insert{1, decimal.new('1337.420')}
---
- [1, 1337.420]
...

```