New Type Checker: Make TypeEnvironment shared

[Vengarioth]

The old TypeEnvironment was encapsulated inside our SymbolTable for convenience, this lead to an unclear API. We first make the TypeEnvironment shared so old code doesn't break:

• Make TypeEnvironment shared like SymbolTable: https://github.com/xshade-lang/xshade/blob/master/src/type_system/symbol_table.rs#L10
• Make SymbolTable use a shared reference to the TypeEnvironment: https://github.com/xshade-lang/xshade/blob/master/src/type_system/symbol_table.rs#L71