Implement Glob.matchAsPrefix()

This also allows matchesFull() to be used with Glob patterns.
Add center to quiver.strings
Add Glob.matchAsPrefix() to glob test.
Pad extra char on right instead of left per ruby
Add quiver.streams.concat
Fix HashBiMap to match Map<K, V> spec
Shorthand syntax for putIfAbsent
Make createTimer assignable to CreateTimer and add tests
Improve docs
Remove expectAsync usage
Add duration unit constants to quiver.time
First-stab at AVL tree set.
Reviewer comments
More cleanup.
Add .from() -> Iterator<V>
BugFix: Make bidirectional iterators work.

TreeSet was returning Iterator<V> for iterator and reversed, which didn't
break the contract with Iterable<V>; however it should have been returning
bidirectional iterators, which was clearly not working.

Added tests for noraml iterators as well as new from() iterator.
Moving tests to "treeset_test.dart"

Just clean up, no code change.
Reviewer Comments

reversed -> reverseIterator
from -> fromIterator
Comment clarifications
Reduce externally visible api.
Perf: improve union, intersection, and difference if both are TreeSets.
Merge pull request #93 from seaneagan/duration_unit_constants

Add duration unit constants to quiver.time
Merge pull request #89 from seaneagan/fix_create_timer

Fix CreateTimer by adding Duration param and add tests