Table extension calls arena allocator functions directly

[pyrmont]

My understanding from the man page is that cmark-gfm supports custom allocators by providing an API for calling functions of the memory allocator. However, table.c contains calls to functions defined in arena.c that are called regardless of the allocator provided.

I don't know the cmark-gfm codebase well enough to be sure but is this behaviour intentional? I was trying to write a custom allocator to work around the issue described in #207 when I noticed this.