Support `TealType` subroutine annotations

## Summary

Some developers use `TealType` annotations in subroutines to declare the expected type of an argument or return value (uint64, bytes, or any). For example:

```python
from pyteal import *

@Subroutine(TealType.uint64)
def mySubroutine(a: TealType.uint64, b: TealType.uint64) -> TealType.uint64:
return a + b
```

Unfortunately these types are not truthful -- the actual arguments and return values must be `Expr`s. So it would be beneficial to be able to express the same constraints, but in a way that preserves the actual type information.

## Scope
Somehow allow `TealType` declarations in a way that does not break the actual Python types.

* One possible solution to this would be to make `Expr` a generic type that takes a `TealType` as a parameter, such as `Expr[TealType.uint64]`. (Note that this would likely require changes to all subclasses of `Expr` so that they can properly handle the fact that it is now a generic type.) Then the example could turn into:
```python
@Subroutine(TealType.uint64)
def mySubroutine(a: Expr[TealType.uint64], b: Expr[TealType.uint64]) -> Expr[TealType.uint64]:
return a + b
```

Regardless of how the `TealType` of arguments and return values are declared, the compiler should also perform type checking every time a subroutine is invoked to verify that the argument being passed to it conform to declared types.