Stack overflow on deeply nested LaTeX input

The parser and layout engine in affected versions of `latex-rust` recurse once per level of nesting with no limit. Deeply nested input, such as `\frac{1}{…}`, braces, `\sqrt{…}`, `\left(`, or superscripts nested hundreds or thousands of levels deep, exhausts the call stack. A stack overflow in Rust aborts the process. It is not a panic, so it cannot be caught with `catch_unwind`. A program that renders LaTeX from untrusted input can therefore be terminated by a short string. With a release build on an 8 MiB stack, 700 nested `\frac{1}{…}` (about 7 KB of input) abort version 1.0.4, and 10,000 nested braces (about 20 KB) abort it as well. The threshold is lower on smaller stacks and in debug builds; on a 2 MiB thread in a debug build, about 35 nested fractions are enough. ```rust let mut src = String::from("1"); for _ in 0..700 { src = format!("\\frac{{1}}{{{src}}}"); } let _ = latex_rust::parse(&src); // stack overflow, process aborts ``` The fixed versions count nesting depth in the parser and in `layout` and refuse input that nests more than 32 levels. `parse`, `parse_with_colors`, and the `latex_to_*` functions return `ParseError::Malformed("input nests deeper than 32 levels")`, and `layout` returns `Error::Unsupported { what: "tree nests deeper than 32 levels" }` for a tree built by hand. The count is of parser recursion levels, and a braced argument costs two, so the limit admits 15 nested `\frac`, `\sqrt`, or `x^{…}`, and 31 nested groups, `\left…\right` pairs, or enviro