feat: Index exercises by name

BREAKING CHANGE: This changes the way you use `rustlings run` by now
requiring an abridged form of the previous filename, e.g:

`rustlings run exercises/if/if1.rs` becomes
`rustlings run if1`

tests/fixture/success/info.toml

@@ -1,7 +1,9 @@
 [[exercises]]
+name = "compSuccess"
 path = "compSuccess.rs"
 mode = "compile"
 
 [[exercises]]
+name = "testSuccess"
 path = "testSuccess.rs"
-mode = "test"
+mode = "test"