Add cargo xtask msrv (#3189)

* added cargo xtask msrv X.Y placeholder function

* add placeholder function cargo xtask msrv (X.Y)

* implement CLI arg vec, graceful errors, formatting

* impl. xtask msrv insert,upsert,graceful failures

* misc fixes - still need to update write method

* interim commit before removing comments

* updated per @djmitche suggestions. 2 fixes todo

* impl unwrap error checks, file truncating, etc

* removed test paths

* updated toml with specific regex version

* updated cargo toml and lock for regex version

* updated cargo toml and lock for regex version

* fixed clippy complaints

* impl replace file content, not comments

* accept djmitche suggestion to change comment documentation

Co-authored-by: Dustin J. Mitchell <dustin@v.igoro.us>

* refactored, need to fix Cow return

* removed debug prints, unncessary if, etc.

* fix non-idiomatic rust, arg.len error handling

* add #MSRV to config files and test

---------

Co-authored-by: iwyatt <sayhello+git@isaacwyatt.com>
Co-authored-by: Dustin J. Mitchell <dustin@v.igoro.us>

.github/workflows/checks.yml

@@ -29,9 +29,7 @@ jobs:
 
       - uses: actions-rs/toolchain@v1
         with:
-          # Fixed version for clippy lints.  Bump this as necesary.  It must not
-          # be older than the MSRV in tests.yml.
-          toolchain: "1.64"
+          toolchain: "1.64" # MSRV
           override: true
 
       - uses: actions-rs/cargo@v1.0.3
@@ -100,8 +98,7 @@ jobs:
 
       - uses: actions-rs/toolchain@v1
         with:
-          # Same as the MSRV in rust-tests.yml
-          toolchain: "1.64"
+          toolchain: "1.64" # MSRV
           override: true
 
       - uses: actions-rs/cargo@v1.0.3