Continue from the previous result when a line starts with an operator

[ChrisJr404]

This implements #306: if a line starts with a binary operator that can't begin an expression on its own, fend continues from the previous result, the way a pocket or on-screen calculator does.

> 5 * 10
50
> * 2
100
> to hex
64

Under the hood, when a previous result is available in _, an input whose first token is an operator that needs a left-hand operand (*, ^, mod, to/as, bit shifts, bitwise ops, ...) is rewritten to prepend _. Operators that also have a prefix form (+, -, /) are deliberately left alone, so -5 and /2 still mean negative five and one half. Nothing changes when there is no previous result yet, so a leading operator in a fresh context is still an error as before.

I added integration tests covering the rewritten operators, chaining (* 2 then ^ 2), the interaction with a leading @ attribute, the untouched -// cases, and the no-previous-result case, and documented the behaviour in the expressions chapter. cargo test, cargo fmt --check and cargo clippy --all-targets are all clean.

[ChrisJr404]

The ubuntu job didn't fail on a test, it timed out in the "Test (Linux i686)" step while apt-get was installing gcc-i686-linux-gnu (the download of cpp-13-i686-linux-gnu stalled on the azure mirror and the job hit the 20 minute limit). clippy, fmt and the macos/windows builds all passed, so this looks like a transient mirror hiccup rather than anything in the change. Could you re-run that job when you get a chance?