Adds Compare And Swap to Hyperbee

[utanapishtim]

Adds the ability to compare and swap on a key in a Hyperbee. In particular, if we have:

• a hyperbee: bee
• a key: key
• a value we want to set: value
• the current value in bee at key: current
• a value we want to compare against current: test
• and a predicate that will compare test and the current value in bee at key: compare

Then Hyperbee.cas() compares current and test through compare and:

if true: sets key at value in bee and returns current
if false: return null

The default compare function encodes test through bee.valueEncoding and compares against the encoded value of current.

[utanapishtim]

@mafintosh do I need to grab the lock for this like in _put?

[mafintosh]

1.9.0