Replace lazycell with once_cell

We started to use lazycell because syntect already used it. But syntect has
changed to use once_cell. So we should also do that to prepare for using the
upcoming version of syntect.

Cargo.toml

@@ -49,7 +49,7 @@ bincode = "1.0"
 console = "0.15.0"
 flate2 = "1.0"
 lazy_static = { version = "1.4", optional = true }
-lazycell = "1.0"
+once_cell = "1.8"
 thiserror = "1.0"
 wild = { version = "2.0", optional = true }
 content_inspector = "0.2.4"