rust: move all crates in the main workspace to edition 2021

We've changed our minimum Rust version to 1.61.0 in the previous patch,
and edition 2021 predates that version.

rust/hg-core/Cargo.toml

@@ -3,7 +3,7 b' name = "hg-core"'
 version = "0.1.0"
 authors = ["Georges Racinet <[e-mail]>"]
 description = "Mercurial pure Rust core library, with no assumption on Python bindings (FFI)"
-edition = "2018"
+edition = "2021"
 
 [lib]
 name = "hg"

rust/hg-cpython/Cargo.toml

@@ -2,7 +2,7 b''
 name = "hg-cpython"
 version = "0.1.0"
 authors = ["Georges Racinet <[e-mail]>"]
-edition = "2018"
+edition = "2021"
 
 [lib]
 name='rusthg'

rust/rhg/Cargo.toml

@@ -5,7 +5,7 b' authors = ['
     "Antoine Cezar <[e-mail]>",
     "Raphaël Gomès <[e-mail]>",
 ]
-edition = "2018"
+edition = "2021"
 
 [dependencies]
 atty = "0.2.14"