Reduce calls to malloc in map

[bkthomps]

Using this function for testing:

    map me = map_init(sizeof(int), sizeof(double), compare_int);
    int count = 0;
    int i;
    for (i = 0; i < 1000000; i++) {
        double number = i + 0.5;
        map_put(me, &i, &number);
    }
    for (i = 0; i < 1000000; i++) {
        double get;
        map_get(&get, me, &i);
        assert(get < i + 0.6);
        assert(get > i + 0.4);
        count += map_contains(me, &i);
    }
    printf("%d\n", count);
    map_destroy(me);

The old version would take 0.97s and the new version takes 0.6s. This is an improvement of 40%.

[codecov-commenter]

Codecov Report

Merging #101 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #101   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        16           
  Lines         2741      2763   +22     
=========================================
+ Hits          2741      2763   +22     

Impacted Files	Coverage Δ
src/map.c	100.00% <100.00%> (ø)

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