mpl: add mpl_hash

[hzhou]

Pull Request Description

Add mpl_hash, a string to string hash.

uthash is cumbersome to use as a string to string hash. It requires users to define the hash struct, and manage key and value memory allocations, and it is not good in performance due to fragmented memory allocations.

This PR adds MPL_hash, a specialized string to string hash utility. It manages key and value memories using a dynamically managed memory slab for better efficiency. The strings are dynamically added but only freed at MPL_hash_free. To erase a key, simply set that key to value NULL.

Comparison in usage:

/* uthash */
#include "uthash.h"

struct my_struct {
    char *key;
    char *value;
    UT_hash_handle hh;
};

void main(void)
{
    const char *key = "name";
    const char *val = "value";
    struct my_struct *hash = NULL;

    struct my_struct *s;
    s = (struct my_struct *) MPL_malloc(sizeof(*s), MPL_MEM_OTHER);
    s->key = strdup(key);
    s->value = strdup(val);
    HASH_ADD_KEYPTR(hh, hash, s->key, strlen(s->key), s, MPL_MEM_OTHER);

    HASH_FIND_STR(hash, "name", s);
    if (s) {
        printf("Found in hash: %s -> %s\n", s->key, s->value);
    }

    struct my_struct *tmp;
    HASH_ITER(hh, hash, s, tmp) {
        MPL_free(s->key);
        MPL_free(s->value);
        HASH_DEL(hash, s);
        MPL_free(s);
    }
}

/* mpl_hash */
#include "mpl_hash.h"

void main(void)
{
    const char *key = "name";
    const char *val = "value";
    struct MPL_hash *hash = NULL;

    hash = MPL_hash_new();

    MPL_hash_set(hash, key, val);

    char *s = MPL_hash_get(hash, key);
    if (s) {
        printf("Found in hash: %s -> %s\n", key, s);
    }

    MPL_hash_free(hash);
}

Performance-wise, adding 1 million entries with 1000 unique keys, measured on my workstation using time:

    uthash:   0.234s
    mpl_hash:  0.095s

[skip warnings]

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