Add Buf_read.BE/LE parsers

[Cjen1]

This PR adds the relevant parsers for integer primitives, equivalent to Buf_write.BE/LE to Buf_read.

It resolves #390.

Completed portions:

• Big endian parsers
• Big endian tests/fuzzers
• Little endian parsers
• Little endian tests/fuzzers
• uint48 parsers

[talex5]

Looks good so far!

[Cjen1]

@talex5 This should be a completed PR.

I think that the uint48 implementation may not be fully correct but I'm testing to make sure now.
(I think Int64.of_int32 does not preserve the msb)

[Cjen1]

I think that the uint48 implementation may not be fully correct but I'm testing to make sure now. (I think Int64.of_int32 does not preserve the msb)

It does not preserve the sign (or rather it preserves the value rather than the bits).
This is now fixed by doing everything in the int64 representation.

[talex5]

Looks right to me, but some simplifications/speedups are possible I think.

[talex5]

This might be better (haven't benchmarked it though):

Suggested change

	let lower_16 = Bigstringaf.get_int16_le t.buf t.pos |> Int64.of_int in
	let middle_16 = Bigstringaf.get_int16_le t.buf (t.pos + 2) |> Int64.of_int in
	let lower_32 = Bigstringaf.get_int32_le t.buf t.pos |> Int64.of_int |> Int64.log_and 0xffffffffL in

Then the total is just Int64.(logor lower_32 (shift_left upper_16 32)).

[Cjen1]

I'm not sure this is correct for -1L since the Int64.of_int32 moves the sign bit such that the value is the same (but not the bits).
This was the correctness bug I was concerned about previously.

logor may be faster though than just adding.

[talex5]

It doesn't move the sign bit, it just replicates it into the extra bits (which is correct for signed numbers but not for unsigned ones). The logand gets rid of the extra copies:

# Int64.of_int32 0xe7654321l |> Printf.sprintf "0x%Lx";;
- : string = "0xffffffffe7654321"
# Int64.of_int32 0xe7654321l |> Int64.logand 0xffffffffL |> Printf.sprintf "0x%Lx";;
- : string = "0xe7654321"

[Cjen1]

Good to know! I've left the fuzz model using the 3 * 16 bit one for validation.

[talex5]

Thanks! I squashed and rebased it on top of #402 to avoid needing the Stdlib. prefix.