Some design notes for the Rust-lite semantics

[virgil-serbanuta]

These notes are not meant to describe the definite version of the Rust-lite semantics, especially for the execution part, which is obviously incomplete, and whose configuration is just a sketch (although it may be a fairly good one).

However, these notes are meant to describe what I intended to do for this semantics, and the intent of this PR is to get approval for the direction in which this semantics will go.

These notes are out of sync with the current implementation of "indexing", which should have been called "preprocessing" anyway. I plan to fix that, unless I should change focus to a different semantics (e.g. Solidity-lite).

[ACassimiro]

@virgil-serbanuta thank you for adding this documentation! This reflects the description of the semantics and adaptations that we're performing in order to implement Rust-lite, is it correct? I think it looks really good and for providing the directions on what has been implemented so far and the approach used for it. One thing that I think would be complimentary to this and valuable for the project README is the instructions on running the semantics, as well as a brief overview on what the commands provided in the project's Makefile do. Not sure if this is pertinent to this PR, though.

Furthermore, some minor notes on the text:

• Line 22, typo byß;
• Line 34, I believe 'int' is used where 'in' should be;

[virgil-serbanuta]

[...] One thing that I think would be complimentary to this and valuable for the project README is the instructions on running the semantics, as well as a brief overview on what the commands provided in the project's Makefile do. Not sure if this is pertinent to this PR, though.

I added some instructions, please tell me if I should do more.