Add extract expressions

[jasonpaulos]

Adds expressions for using new TEAL 5 extract opcodes.

[algochoi]

The code looks good to me, just had two questions:

1. Will there be a way to force the substring or extract TEAL op code using Pyteal (i.e. the ones that take immediates)? I guess there isn't much of a usecase right now, but those ops do save a couple bytes (assuming they are uint8) if I recall correctly.
2. There is a somewhat hidden shortcut in extract where if you specify the length to be 0 as an immediate, it extracts until the end of the string. I think this is linked to point 1 and wondering if it should be supported.

[jasonpaulos]

I agree it would be nice if there was a way to generate the substring and extract opcodes, since they are more efficiently encoded.

Normally if an operation can accept either a stack value or an immediate, PyTeal allows Union[int, Expr] and internally chooses the correct opcode based on the arguments. However, with these I think a better approach would be to have some sort of optimization that checks if the input expressions can be transformed into integers, and if that's the case and the integers aren't too large, then the immediate form of the opcode is used. Furthermore, in most cases it will probably be more efficient to use extract over substring, so the optimization could also make this happen when it's beneficial.

For extract the length arg being 0 indicates the end of the string, but for extract3 there is no equivalent to this, correct? I.e. if I pass in 0 as the length arg, I will get a 0 length substring. If this is the case, that's a constraint we'll need to keep in mind when writing an optimization for this.

Since there's a fair bit of complexity here, I think it would be better to address this in a separate issue.

edit: I made an issue: #119

[algochoi]

I think that sounds good -- Yes the 0 length trick will only work with the immediates extract opcode rather than the extract3 op code.

[algochoi]

LGTM 🚢