refactor(core/ethereum): improve API of the rlp module

[matejcik]

• improve length calculation so that we don't need to actually encode to get the length
• instead of returning bytes, directly write into a provided Writer

the test vectors in test_trezor.crypto.rlp.py, and the Ethereum device tests, show that the new implementation produces results identical to the old one

[prusnak]

This seems weird. Maybe you meant:

Suggested change

	if x < 0xFF ** exp:
	if x < 0x100 ** exp:

Also this function returns 0 for x=0. Not sure if that is the intention.

[matejcik]

Also this function returns 0 for x=0. Not sure if that is the intention.

This is intentional, yes

But good point about the equality test. I'll need to add an edge value to the testcases first as it's apparently not covered.

[matejcik]

3d4d186

[mmilata]

One question, otherwise looks better and more efficient than the original version.

[mmilata]

What does the recursion do here?

Also, should there be bytearray since it's checked against in the code?

[matejcik]

Ah, I meant to run this though mypy and forgot. Good catch, this is supposed to be something like Union[list["RLPItem"], bytes, int]

(also I suspect that mypy won't actually allow a recursive type)

[matejcik]

4a009c8