gypsum: Introduce ModuleScope for per-file bindings

ModuleScope is a scope for each file in a package. Global definitions
and imports are bound in ModuleScope initially. Definitions (but not
imports) are copied to GlobalScope after we have walked the AST.

This means that symbols that are imported in one file are not visible
in other files. Imported symbols shadow definitions in other files but
they are overloaded with definitions in the same file.

Fixes #34.

gypsum/ir.py

@@ -475,13 +475,17 @@ class Global(IrTopDefn):
         id (DefnId): unique identifier for the global.
         sourceName (str?): name of the definition in source code
         astDefn (ast.Node?): the location in source code where the global is defined.
+        astVarDefn (ast.Node?): the location in source code of the variable definition
+            containing the field definition.
         type (Type?): the type of the global. May be `None` before type analysis.
         flags (frozenset[flag]): flags indicating how this global may be used. Valid flags are
             `EXTERN`, `LET`, `PUBLIC`.
     """
 
-    def __init__(self, name, id, sourceName=None, astDefn=None, type=None, flags=frozenset()):
+    def __init__(self, name, id, sourceName=None, astDefn=None, astVarDefn=None,
+                 type=None, flags=frozenset()):
         super(Global, self).__init__(name, id, sourceName, astDefn)
+        self.astVarDefn = astVarDefn
         self.type = type
         self.flags = flags
 
@@ -1089,9 +1093,10 @@ def findCommonUpperBound(self, other):
 PARAMETER = "parameter"
 
 class Variable(IrDefinition):
-    def __init__(self, name, sourceName=None, astDefn=None, type=None, kind=LOCAL,
-                 flags=frozenset()):
+    def __init__(self, name, sourceName=None, astDefn=None, astVarDefn=None,
+                 type=None, kind=LOCAL, flags=frozenset()):
         super(Variable, self).__init__(name, sourceName, astDefn)
+        self.astVarDefn = astVarDefn
         self.type = type
         self.kind = kind
         self.flags = flags
@@ -1117,6 +1122,8 @@ class Field(IrDefinition):
         name (Name): the name of the field.
         sourceName (str?): name of the definition in source code
         astDefn (ast.Node?): the location in source code where the field is defined.
+        astVarDefn (ast.Node?): the location in source code of the variable definition
+            containing the field definition.
         type (Type?): the type of the field. May be `None` before type analysis.
         flags (frozenset[flag]): flags indicating how this field is used. Valid flags are
             `LET`, `PUBLIC`, `PROTECTED`, `PRIVATE`, `STATIC`.
@@ -1127,9 +1134,10 @@ class that inherits this field).
             instructions. This will usually be `None` before semantic analysis.
     """
 
-    def __init__(self, name, sourceName=None, astDefn=None, type=None, flags=frozenset(),
-                 definingClass=None, index=None):
+    def __init__(self, name, sourceName=None, astDefn=None, astVarDefn=None, type=None,
+                 flags=frozenset(), definingClass=None, index=None):
         super(Field, self).__init__(name, sourceName, astDefn)
+        self.astVarDefn = astVarDefn
         self.type = type
         self.flags = flags
         self.definingClass = definingClass