improve Decimal conversion performance using `_mantissa` (#124)

* improve Decimal conversion performance using `_mantissa`

* fix linux builds by keeping the decimal string init in a compiler directive

* more efficient zero out assurance after buffer creation

* Decimal (from bigint) initializer

Decimal and Foundation (#122)

* add CustomDebugStringConvertible

* reduce Foundation dependency

* if foundation is available, add Decimal functionality

* more concise floating point init

* dry up data initialization

Revert docc related changes from "Migrate from xcworkspace to swift p…

…ackage (#119)"

This reverts the docc related changes in commit 5f01bdc.

fix links

Bumped version

re-enabled Mac Catalyst