Raise minimum supported compiler to rustc 1.46

Required by the use of Option::zip.

    error[E0658]: use of unstable library feature 'option_zip'
       --> src/loc.rs:551:14
        |
    551 |             .zip(self.expansion_loc.as_ref())
        |              ^^^
        |
        = note: see issue #70086 <rust-lang/rust#70086> for more information

    error[E0658]: use of unstable library feature 'option_zip'
       --> src/loc.rs:529:36
        |
    529 |             spelling_included_from.zip(expansion_included_from).map_or(
        |                                    ^^^
        |
        = note: see issue #70086 <rust-lang/rust#70086> for more information

Author: dtolnay

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [beta, stable, 1.45.0]
+        rust: [beta, stable, 1.46.0]
     steps:
       - uses: actions/checkout@v2
       - uses: dtolnay/rust-toolchain@master