strmap: add more utility functions

This adds a number of additional convienence functions I want/need:
  * strmap_get_size()
  * strmap_empty()
  * strmap_remove()
  * strmap_for_each_entry()
  * strmap_get_entry()

I suspect the first four are self-explanatory.

strmap_get_entry() is similar to strmap_get() except that instead of just
returning the void* value that the string maps to, it returns the
strmap_entry that contains both the string and the void* value (or
NULL if the string isn't in the map).  This is helpful because it avoids
multiple lookups, e.g. in some cases a caller would need to call:
  * strmap_contains() to check that the map has an entry for the string
  * strmap_get() to get the void* value
  * <do some work to update the value>
  * strmap_put() to update/overwrite the value
If the void* pointer returned really is a pointer, then the last step is
unnecessary, but if the void* pointer is just cast to an integer then
strmap_put() will be needed.  In contrast, one can call strmap_get_entry()
and then:
  * check if the string was in the map by whether the pointer is NULL
  * access the value via entry->value
  * directly update entry->value
meaning that we can replace two or three hash table lookups with one.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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