Add par_vec, `ParVec` concurrent vector wrapper
Merge pull request #21 from cybergeek94/master

`ParVec`: A vector that allows concurrent manipulation of non-overlapping slices.
add `compare` module with comparators
parameterize `IntervalHeap` with a comparator
implement `Default` for comparators
rename methods

- s/with_capacity_cmp/with_capacity_and_cmp/
- s/from_vec_cmp/from_vec_and_cmp/
s/inteval_heap_push/interval_heap_push/
use `Default` in `IntervalHeap` constructors
remove type parameters from `Natural`
update `IntervalHeap` docs