Closes #4413

@TarantoolBot document
Title: update operations on decimal fields.

tuple:update and space:update now support deicmal operands for
arithmetic operations ('+' and '-'). The syntax is as usual:
```
d = box.tuple.new(decimal.new('1'))
---
...
d:update{{'+', 1, decimal.new('0.5')}}
---
- [1.5]
...
```

Insertion ('!') and assignment ('=') are also supported:
```
a = decimal.new('1')
---
...
b = decimal.new('1e10')
---
...
c = decimal.new('1e-10')
---
...
d = box.tuple.new{5, a, 6, b, 7, c, "string"}
---
...
d
---
- [5, 1, 6, 10000000000, 7, 0.0000000001, 'string']
...

d:update{{'!', 3, dec.new('1234.5678')}}
---
- [5, 1, 1234.5678, 6, 10000000000, 7, 0.0000000001, 'string']
...
d:update{{'=', -1, dec.new('0.12345678910111213')}}
---
- [5, 1, 6, 10000000000, 7, 0.0000000001, 0.12345678910111213]

When performing an arithmetic operation ('+', '-'), where either the
updated field or the operand is decimal, the result will be decimal.

When both the updated field and the operand are decimal, the result
will, of course, be decimal.

...
```