ssl/crypto uses two allocation idioms for the same job

[SeanTAllen]

ssl/crypto allocates its output buffers two ways.

@pony_alloc(@pony_ctx(), size) then from_cpointer, nine sites:

• ssl/crypto/hash_fn.pony: the eight hash primitives (MD4 through SHA512), each with Array[U8].from_cpointer.
• ssl/crypto/digest.pony: final(), with String.from_cpointer.

Array[U8].init(0, size), three sites:

• ssl/crypto/rand_bytes.pony
• ssl/crypto/hmac_sha256.pony
• ssl/crypto/pbkdf2_sha256.pony

Both allocate a fixed-size buffer for OpenSSL to write into. #77 removed a comment that claimed the two differ for the garbage collector; they do not. It left unifying them as a separate question. This is that question.