add yp_alloc to begin creating our own allocator

[HParker]

This introduces yp_malloc yp_calloc and yp_free as allocation APIs which we can use to manage allocations.

Once these are centralized we can use xmalloc if in a Ruby context and/or use an arena allocator to reduce syscalls to malloc.

Related to this issue: #1337

This is a small part of my working arena allocator here: https://github.com/HParker/yarp/tree/introduce-yp_malloc

In an effort to make smaller changes at a time, this only updates direct malloc/calloc/free calls in yarp.c. There are more allocations that I will move in followup PRs.

[kddnewton]

In general this looks good, but before we merge I'd like to see the functions in yp_alloc.c marked as static that shouldn't be visible to other translation units. Also, we definitely need some comments on all of the functions that are visible beyond yp_alloc.h.

I'm not entirely clear on the USE_ARENA macro. It looks like it's triggering behavior, but it's always set to 1. Is this meant as a migration path? If so I would think it should be set to 0.

[HParker]

Thanks @kddnewton I incorporated your feedback.

I removed the USE_ARENA macro, was there to make comparing performance easier, but that doesn't need to be here now, so I removed it.

How does this look now?

[HParker]

Closing while we discuss direction to reopen later.