Suggest using a temporary variable to fix borrowck errors

In Rust, nesting method calls with both require `&mut` access to `self`
produces a borrow-check error:

    error[E0499]: cannot borrow `*self` as mutable more than once at a time
     --> src/lib.rs:7:14
      |
    7 |     self.foo(self.bar());
      |     ---------^^^^^^^^^^-
      |     |    |   |
      |     |    |   second mutable borrow occurs here
      |     |    first borrow later used by call
      |     first mutable borrow occurs here

That's because Rust has a left-to-right evaluation order, and the method
receiver is passed first. Thus, the argument to the method cannot then
mutate `self`.

There's an easy solution to this error: just extract a local variable
for the inner argument:

    let tmp = self.bar();
    self.foo(tmp);

However, the error doesn't give any suggestion of how to solve the
problem. As a result, new users may assume that it's impossible to
express their code correctly and get stuck.

This commit adds a (non-structured) suggestion to extract a local
variable for the inner argument to solve the error. The suggestion uses
heuristics that eliminate most false positives, though there are a few
false negatives (cases where the suggestion should be emitted but is
not). Those other cases can be implemented in a future change.

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