Decmath

Internally restructured project with some **precision improvements** as a result.

Started to do proper **testing** (See issue 2 for more detail on the current state of this).

**Fixed some issues with trigonometric functions** therefore you should switch to this version immediately.

Upgrading to v0.4.0 from v0.3.x: no changes needed.

Known issues: `gcd(x, y)` doesn't yet support `Decimal` input.

Added:
- `asinh(x)`
- `acosh(x)`
- `atanh(x)`
- `modf(x)`
- `expm1(x)`
- `log1p(x)`

Changed behaviour of:
- `sign(x)`

Added:
- `signt(x)`
- `fabs(x)`
- `erf(x)`
- `erfc(x)`
- `fsum(iterable)`
- `trunc(x)`
- `log2(x)`

And improved some precision issues.