When converting a double larger or equal to math.MaxInt64 to an integer, it previously overflowed silently. It now returns an explicit error.
This fixes along the way, the math.abs(-9223372036854775808) issue.
We now have a literal representation for BLOBs. Any string literal starting
with '\x' is parsed as an hex encoded blob. This mimics PostgreSQL's behavior.
```sql
SELECT '\xAAFF';
```
* Add math.acos and math.abs
* Add acosh, asin, asinh, atan and atan2
* Test for proper function documentation
* Cast to double on math functions
* Use the new types.Value interface
This replaces the Value struct by an interface to allow us to override some
values behavior in the future.
It also introduces a new package types, which contains type definitions, comparison,
and arithmetics.
Concerning encoding, Genji now only uses on type of encoding for values. This simplifies
indexing logic as well as table access in general.
