Improve the bitset

Changes:

• Bitset interface changes.
• Improved complexity of from_array for bitsets.

Added setops

Merge pull request #6 from diku-dk/setops

Add module `setops` with set operations

Universal hashing.

• Changes in API and universal hashing.

v0.7.0

quick fix.

v0.6.5

• Optimize membership.

v0.6.4

• This changes the API size types.
• Uses less memory.

v0.6.3: Merge pull request #4 from diku-dk/lessmem

• Two level hashmaps uses less memory now.

v0.6.1

• Bug fixes to Eytzinger tests.

v0.5.1

• Extended API to have no duplicate variants.

v0.4.1

• Minor changes in API.
• Added more documentation.