Use Uint32/64 to hold unsingned large integers

[andersfugmann]

Add option to use UInt{64,32}.t to hold unsinged integers.
This will depend on an external package "`Uint? " (which depends on stdint).

• Extend options to allow use of `UInt package
• Consider compatibility with JS platform.
• Disable option if UInt is not available at compiletime (Or just alias to Int{64,32}.t