makes filter-keys-by-val 9% faster by using into, similar to other fns in ns

(require 'criterium.core)
(require 'useful.map)
(def my-map (zipmap (range 100) (range 100)))

(criterium.core/bench (useful.map/original-filter-keys-by-val odd? my-map))
Evaluation count : 3415500 in 60 samples of 56925 calls.
             Execution time mean : 17.583266 us
    Execution time std-deviation : 124.371912 ns
   Execution time lower quantile : 17.425613 us ( 2.5%)
   Execution time upper quantile : 17.898538 us (97.5%)

Found 2 outliers in 60 samples (3.3333 %)
  low-severe   2 (3.3333 %)
 Variance from outliers : 1.6389 % Variance is slightly inflated by outliers

(criterium.core/bench (useful.map/filter-keys-by-val-with-into odd? my-map))
Evaluation count : 3778800 in 60 samples of 62980 calls.
             Execution time mean : 16.009860 us
    Execution time std-deviation : 89.880836 ns
   Execution time lower quantile : 15.875754 us ( 2.5%)
   Execution time upper quantile : 16.219088 us (97.5%)

Found 2 outliers in 60 samples (3.3333 %)
  low-severe   2 (3.3333 %)
 Variance from outliers : 1.6389 % Variance is slightly inflated by outliers
makes filter-vals and remove-vals 3x faster

(require 'criterium.core)
(require 'useful.map)

(def my-map (zipmap (range 100) (range 100)))

(criterium.core/bench (useful.map/original-filter-vals my-map odd?))
Evaluation count : 2203320 in 60 samples of 36722 calls.
             Execution time mean : 27.389858 us
    Execution time std-deviation : 206.895390 ns
   Execution time lower quantile : 27.186863 us ( 2.5%)
   Execution time upper quantile : 27.997736 us (97.5%)

Found 3 outliers in 60 samples (5.0000 %)
  low-severe   1 (1.6667 %)
  low-mild   2 (3.3333 %)
 Variance from outliers : 1.6389 % Variance is slightly inflated by outliers

(criterium.core/bench (useful.map/new-filter-vals my-map odd?))
Evaluation count : 6759300 in 60 samples of 112655 calls.
             Execution time mean : 8.949856 us
    Execution time std-deviation : 39.725387 ns
   Execution time lower quantile : 8.882420 us ( 2.5%)
   Execution time upper quantile : 9.032029 us (97.5%)
Merge branch 'filter-vals-speedup' of github.com:RedBrainLabs/useful into develop

Conflicts:
	src/flatland/useful/map.clj