[perf] Eliminate initial soft-stack allocation

[DavidBuchanan314]

per benchmarks https://github.com/DavidBuchanan314/dag-cbor-benchmark we're significantly slower than libipld for very short messages.

This is likely in large part due to the sizeable initial stack allocations we do

dag-cbrrr/src/cbrrr/_cbrrr.c

Lines 483 to 484 in ad56c83

	size_t stack_len = 16;
	DCToken *parse_stack = malloc(stack_len * sizeof(*parse_stack));

dag-cbrrr/src/cbrrr/_cbrrr.c

Lines 1042 to 1043 in ad56c83

	size_t stack_len = 16;
	EncoderStackFrame *encoder_stack = malloc(stack_len * sizeof(*encoder_stack));

We should use a small stack-allocated buffer for the initial stack, then move to heap when we run out of that (which implies a buffer copy, but probably worth it overall? - or maybe we could avoid the copy by treating the heap allocation as "overflow" room)

Ditto for the buffer we decode into? We'll always need at least one allocation, though

dag-cbrrr/src/cbrrr/_cbrrr.c

Line 1364 in ad56c83

buf.buf = malloc(buf.capacity);