feat: Refactor hint system

Hints are now accessible using the CLI subcommand `rustlings hint
<exercise name`.

BREAKING CHANGE: This fundamentally changes the way people interact with exercises.

tests/fixture/failure/info.toml

@@ -2,8 +2,10 @@
 name = "compFailure"
 path = "compFailure.rs"
 mode = "compile"
+hint = """"""
 
 [[exercises]]
 name = "testFailure"
 path = "testFailure.rs"
 mode = "test"
+hint = """"""

tests/fixture/success/info.toml

@@ -2,8 +2,10 @@
 name = "compSuccess"
 path = "compSuccess.rs"
 mode = "compile"
+hint = """"""
 
 [[exercises]]
 name = "testSuccess"
 path = "testSuccess.rs"
 mode = "test"
+hint = """"""