Allow custom diffing algorithm

[cpud36]

I would like to be able to plug-in custom diffing algorithm in case of failure. Currently the dissimilar crate is hardcoded in the Runtime::panic.

The use case I have, is the following. I would like to check llvm ir output with expect, but textual diffs on llvm ir do not make much sense. For example, if one were to add a variable at the beginning of the function, most futher variables would change their names (they are named in the form %1, %2, %3, ...). But there is an awesome llvm-diff tool, that handles renames much better, so on failure I would like to print the llvm-diff output, not the dissimilar one.

I have two concepts of api in mind:

// Expose the plug-in point, or
impl Expect {
  pub fn diff_fn(&mut self, diff: impl FnOnce(...) -> ...); // <- actually boxes the diff fn
  pub fn assert_eq_with_diff(&self, actual: &str, diff: impl FnOnce(...) -> ...);
}

// A low-level api to allow user customization
impl Expect {
  /// Check if this expect is equal to `actual`.
  ///
  /// Returns ExpectError that contains `expected` and `actual` (also holds the rt lock).
  /// You are expected to report the error and drop it - ExpectError panics on drop.
  ///
  /// If UPDATE_XFLAGS=1 is set, updates the expect and returns none.
  pub fn check_eq(&self, actual: &str) -> Option<ExpectError<'_>>;
}

impl ExpectError {
  /// Prints common error message with user-provided diff
  pub fn report_diff(self, diff: &str);
  /// Prints common error message with user-provided diff
  pub fn report_diff(self, diff: &str);
}

I am willing to create PR if it is an acceptable change