Don't use the terms r-value and l-value

[hanna-kruppe]

Currently used in src/expression.md and src/types/inference.md

These terms are compiler jargon at best and not great for explaining things to users, and even inside the compiler we've been moving away from them (e.g., rust-lang/rust#47837).

[steveklabnik]

We did this for the reference: rust-lang/reference#167

RBE should follow that example.

[steveklabnik]

This is a dup of #988