Derive procedural macros for the lender crate

This crate provides a [for_!] function-like macro that can be used to iterate over an IntoLender with a syntax similar to a for loop:

for_!(x in into_lender {
    ...
});

The macro expands to a while let loop that iterates over a Lender obtained from the IntoLender. The full for syntax is supported (patterns, etc.).