Make BigDecimal object embedded (#507)

Fix: #293

BigDecimal is a very good fit for embedded objects, as most of
them are small, and they're all immutable, so they don't need
any resizing.

In most case it results in smaller objects, but also reduces pointer
chasing as well as make these objects faster to free for the GC.

master:

```ruby
>> ObjectSpace.memsize_of(BigDecimal("422343434234234234234234234234423"))
=> 92
>> ObjectSpace.memsize_of(BigDecimal("4223434342342342342342342342344232342423423"))
=> 96
```

This branch:

```ruby
>> ObjectSpace.memsize_of(BigDecimal("422343434234234234234234234234423"))
=> 80
>> ObjectSpace.memsize_of(BigDecimal("4223434342342342342342342342344232342423423"))
=> 160
```

Author: byroot