Merge pull request #43 from scossu/const_get

Make hashmap const in read-only methods.

Add load factor option

It's now possible to set the load factor at compile time using
the HASHMAP_LOAD_FACTOR define:

   cc -DHASHMAP_LOAD_FACTOR=0.75 ...

or by using the hashmap_set_load_factor function

  hashmap_set_load_factor(map, 0.75);

The load factor should be in the range of 50% to 95%.

Various new updates, features, and optimizations

Changes:
- Added xxhash3 hasher function.
- Added _with_hash functions for manually providing a hash to the
  get, set, and delete functions. This makes the hash callback
  that is given to the hashmap_new function optional.
- Added set_grow_by_power which defines how quickly the hashmap
  grows when needed. Default is 1, which mean it doubles each
  time it needs to grow, setting to 2 or 3 will grow by 4 or 8
  respectively. Max 16.
- Default grow_at percentange has been changed from 0.75 to 0.60.
- The hash field now clipped with bitwise AND instead of shifting.
- The compare function is now optional. When not provided, the
  clipped (48-bit) hash becomes the key.
- Code cleanup

Simplify Murmur return value

Merge pull request #25 from arthurep/fix_resize_new_hashmap

use hashmap_new_with_allocator instead of hashmap_new in resize()

Add hashmap_iter example

Merge pull request #11 from thcopeland/master

Allow passing an element-freeing function to hashmap_new

Merge pull request #13 from scossu/const_void_get

Make "key" argument of hashmap_get const void.

Added hashmap_new_with_allocator

Deprecated hashmap_set_allocator

See #6 for more information

Fix overflow regression

This commit fixes a regression introduced in c039c88 (v0.3.1)
which causes a heap-buffer-overflow during hashmap_free.

fixes #10