feat!(pyargus): simplify the API surface

- Get rid of helper functions. It is not that much more verbose to
  create signals with `argus.FloatSignal(...)` than
  `argus.signal(..., dtype=argus.dtype.float64`).

- Make the package hierarchy flat: everything is under `argus`. If this
  is an issue, it can be changed in the future.

- Add type hints for interval types.

pyargus/argus/__init__.pyi

@@ -0,0 +1,38 @@
+from typing_extensions import TypeAlias
+
+from argus._argus import Abs as Abs
+from argus._argus import Add as Add
+from argus._argus import Always as Always
+from argus._argus import And as And
+from argus._argus import BoolExpr as BoolExpr
+from argus._argus import BoolSignal as BoolSignal
+from argus._argus import Cmp as Cmp
+from argus._argus import ConstBool as ConstBool
+from argus._argus import ConstFloat as ConstFloat
+from argus._argus import ConstInt as ConstInt
+from argus._argus import ConstUInt as ConstUInt
+from argus._argus import Div as Div
+from argus._argus import Eventually as Eventually
+from argus._argus import Expr as Expr
+from argus._argus import FloatSignal as FloatSignal
+from argus._argus import IntSignal as IntSignal
+from argus._argus import Mul as Mul
+from argus._argus import Negate as Negate
+from argus._argus import Next as Next
+from argus._argus import Not as Not
+from argus._argus import NumExpr as NumExpr
+from argus._argus import Or as Or
+from argus._argus import Signal as Signal
+from argus._argus import Trace as Trace
+from argus._argus import UnsignedIntSignal as UnsignedIntSignal
+from argus._argus import Until as Until
+from argus._argus import VarBool as VarBool
+from argus._argus import VarFloat as VarFloat
+from argus._argus import VarInt as VarInt
+from argus._argus import VarUInt as VarUInt
+from argus._argus import dtype as dtype
+from argus._argus import eval_bool_semantics as eval_bool_semantics
+from argus._argus import eval_robust_semantics as eval_robust_semantics
+from argus._argus import parse_expr as parse_expr
+
+AllowedDtype: TypeAlias = bool | int | float