Add python like Sliceable[start:end] operator to vyper

[Philogy]

Simple Summary

Instead of typing out slice(my_sliceable_var, start, length) I'd like to be able to use slice syntax i.e. my_sliceable_var[start:end]. It should have the same behavior and be bounds-checked.

An alternative would be to implement Rust's slicing syntax [start..end] and [start..=end]. What's nice about Rust's approach is that gives syntatic sugar to differentiate between exclusive and inclusive ranges which can be nice. Reused it could serve as syntax sugar for range in the future.

Motivation

The slice builtin leads to some relatively verbose syntax. Square brackets are widely recognized across several languages to represent slicing and will be familiar to most people. Both Solidity & Python support the slicing operator in the form of [start:end]. Rust is quite popular too so [start..end] will be familiar to a lot of devs too.

Specification

TBD.

Backwards Compatibility

Backwards compatible as this wasn't valid syntax before.

Dependencies

None afaik.

References

Copyright

Copyright and related rights waived via CC0

[charles-cooper]

i've been thinking about this for awhile and it would be good to replace the slice builtin. the nice thing is it gives us a way to fix the current API ugliness of slice() (iirc, it is very frustrating that it reverts when the requested length is larger than the length of the provided bytestring, this usually comes up during bytestring building routines)

[charles-cooper]

ah, dup of #3562 (but i left the specification as a stub)

[charles-cooper]

related: #3560