Add spec for CALLER opcode

[ed255]

I have followed the pattern from #83 but using the CallContext in the rw_table instead of the Block table for the value lookup.

Related issue: #86

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I've added two questions for doubts I had:

    # QUESTION: Where is it checked that address raw data is 160 bit length (20
    # bytes)?
    # QUESTION: Which is better of the two options?
    # a. rlc_decode(stack_value) == call_context_lookup()
    # b. stack_value == rlc_encode(call_context_lookup())

[han0110]

Where is it checked that address raw data is 160 bit length (20 bytes)

The address from call context will always be verified by the step that triggers the call (e.g. BeginTx, *CALL*, CREATE*), and because rw_table guarantee read/write consistency, so we don't need to double-check if it's a valid address.

Which is better of the two options?

Option b. sounds better IMHO, since we read the call context first, and do a base conversion (from 256 to r), then push to stack. But both work for me.

[CPerezz]

LGTM modulo applying @han0110 's replies.

[han0110]

LGTM!

[ed255]

Option b. sounds better IMHO, since we read the call context first, and do a base conversion (from 256 to r), then push to stack. But both work for me.

I've updated the spec to follow option b, and removed the notes about checking the address byte length.