Feature: allow setting custom capacity

[koutoftimer]

Pushed changes upsteam.

In my use case, I know maximum amount of keys/buckets of the map and I wanted a way to set capacity.

I ended up supporting both:

• at compilation time via HASHMAP_DEFAULT_CAPACITY
• at runtime as optional .capacity = <value> argument to hashmap_create.

[koutoftimer]

It is possible to move HASHMAP_DEFAULT_CAPACITY to header file in order to explicitly set .capacity to HASHMAP_DEFAULT_CAPACITY and prevent zero initialization ambiguvity.

#ifndef HASHMAP_DEFAULT_CAPACITY
#  define HASHMAP_DEFAULT_CAPACITY 20
#endif

#define hashmap_create(opts) \
        hashmap_create_impl((struct hashmap_create_opts){ \
          .capacity = HASHMAP_DEFAULT_CAPACITY,           \
           opts                                           \
        })

It allows to set

	m->capacity = opts.capacity;

without any additional logic.

UPD: but, on the other hand, compilers may argue about

warning: initializer overrides prior initialization of this subobject [-Winitializer-overrides]  // clang
warning: initialized field overwritten [-Woverride-init] // gcc

or something like that, as soon as you'll try to override .capacity value. clang-20 does, but gcc-14 - doesn't.. at least with default compilation flags (but with -Wall -Wextra it does too).