calyx-py: Remove mutable global state for position tracking

[sampsyo]

The calyx-py AST library tracks (Python) source positions using mutable global state. There are global configuration variables that control the tracking:

calyx/calyx-py/calyx/py_ast.py

Lines 8 to 16 in 85968c4

	"""
	The base path that fileinfos will be relative with respect to. If None, absolute paths will be used.
	"""
	FILEINFO_BASE_PATH = None
	"""
	Determines if sourceinfo and @pos attributes should be emitted or not.
	"""
	EMIT_SOURCELOC = True

And a couple of global tables that store the actual data in class variables, like this:

calyx/calyx-py/calyx/py_ast.py

Lines 36 to 37 in 85968c4

	FileTable.table[filename] = FileTable.counter
	FileTable.counter += 1

Both of these can cause classic global-mutable-state problems with testability and so on. It would be better to associate all of these with the root Program class instead. This way, it would be possible to construct two different Programs from the same Python code and not have them clobber each other.