Reduce calls to malloc in multimap

[bkthomps]

Using the following code:

    multimap me = multimap_init(sizeof(int), sizeof(double), compare_int,
                                compare_int);
    int count = 0;
    int i;
    for (i = 0; i < 1000000; i++) {
        int j;
        for (j = 0; j < 5; j++) {
            multimap_put(me, &i, &j);
        }
    }
    for (i = 0; i < 1000000; i++) {
        int j = 0;
        int get;
        multimap_get_start(me, &i);
        while (multimap_get_next(&get, me)) {
            if (get == j) {
                count++;
            }
            j++;
        }
    }
    printf("%d\n", count);
    multimap_destroy(me);

The old version would run in 2.8s and the new version runs in 1.7s. This is an improvement of 40%.

[codecov-commenter]

Codecov Report

Merging #102 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #102   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        16           
  Lines         2763      2804   +41     
=========================================
+ Hits          2763      2804   +41     

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

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