Add a cap on ensure macro recursion depth

[dtolnay]

Before:

error: recursion limit reached while expanding `$crate::__parse_ensure!`
  --> tests/test_ensure.rs:50:22
   |
50 |       let test = || Ok(ensure!(
   |  ______________________^
51 | |         false | false | false | false | false | false | false | false | false |
52 | |         false | false | false | false | false | false | false | false | false |
53 | |         false | false | false | false | false | false | false | false | false |
...  |
57 | |         false | false | false | false | false | false | false | false | false
58 | |     ));
   | |_____^
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`test_ensure`)
   = note: this error originates in the macro `$crate::__parse_ensure` (in Nightly builds, run with -Z macro-backtrace for more info)

After: we recurse to a maximum of 120 macro calls before bailing out and rendering a fallback message without the lhs/rhs.