Refactor output of OpenQASM 3 exporter to use fewer aliases (#10249)

* Refactor output of OpenQASM 3 exporter to use fewer aliases

This removes spurious `_loose_bit` "registers" from the OpenQASM 3
output, and instead emits loose bits with individual `bit` and
`qubit` declarations.  Non-overlapping registers are emitted using
regular `bit[n]` and `qubit[n]` definitions when possible, and we only
resort to aliasing if we must to describe the structure.

This avoids introducing structure to the definitions that does not exist
in the original program, making round-trips and interactions with other
OQ3 consumers more straightforwards.  It's better not to use advanced
features that don't map to hardware particularly well when it's not
necessary.

On the technical side, all bits are now properly tracked in the symbol
table.  Previously, there was a lot of code duplication, internal state
tracking and magic inferences that attempted to "guess" how a
qubit/clbit should be referred to.  Instead, we just properly add them
as variables to the symbol table, which also drastically reduces the
number of objects that effectively reserve names that the user may not
use.

* Remove unnecessary empty init

* Fix typo

Co-authored-by: Ian Hincks <ian.hincks@ibm.com>

* Add explicit test of old option

---------

Co-authored-by: Ian Hincks <ian.hincks@ibm.com>

qiskit/qasm3/ast.py

@@ -110,9 +110,6 @@ class QuantumInstruction(ASTNode):
         | quantumBarrier
     """
 
-    def __init__(self):
-        pass
-
 
 class ClassicalType(ASTNode):
     """Information about a classical type.  This is just an abstract base for inheritance tests."""
@@ -135,6 +132,10 @@ def __init__(self, size: Optional[int] = None):
         self.size = size
 
 
+class BitType(ClassicalType):
+    """Type information for a single bit."""
+
+
 class BitArrayType(ClassicalType):
     """Type information for a sized number of classical bits."""
 
@@ -332,9 +333,9 @@ class AliasStatement(ASTNode):
         : 'let' Identifier EQUALS indexIdentifier SEMICOLON
     """
 
-    def __init__(self, identifier: Identifier, concatenation: List[Identifier]):
+    def __init__(self, identifier: Identifier, value: Expression):
         self.identifier = identifier
-        self.concatenation = concatenation
+        self.value = value
 
 
 class QuantumGateModifierName(enum.Enum):