Name conflict if Result exists

[tropf]

Dear Maintainer,

if there is a type called Result in the same scope as the macro invocation to impl_context!, the compilation fails as of now (with a rather cryptic error):

90 | impl_context!(ErrorWithContext(SimpleErr));
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `std::fmt::Error`, found `error::Error`
   |
   = note: expected signature `fn(&ErrorWithContext, &mut Formatter<'_>) -> std::result::Result<_, std::fmt::Error>`
              found signature `fn(&ErrorWithContext, &mut Formatter<'_>) -> std::result::Result<_, error::Error>`
   = note: this error originates in the macro `impl_context` (in Nightly builds, run with -Z macro-backtrace for more info)
help: change the output type to match the trait
  --> /home/s9105947/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/thiserror-context-0.1.2/src/lib.rs:303:63
   |
303-             fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), core::fmt::Error> {
303+             fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error> {
   |

error[E0277]: `?` couldn't convert the error to `error::Error`
  --> src/error.rs:90:1
   |
90 | impl_context!(ErrorWithContext(SimpleErr));
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   | |
   | this can't be annotated with `?` because it has type `Result<_, std::fmt::Error>`
   | the trait `From<std::fmt::Error>` is not implemented for `error::Error`

This PR (1) adds a test case for this, and (2) fixes it by referring to core::result::Result instead of only Result.

This is my first time working with macros, and I just followed my gut with the fix. It works, but if it goes against best practices please let me know.