charm: prepare for adding bundle support

Mechanical branch with the following renames:

	Bundle -> CharmArchive
	Dir -> CharmDir
	Read -> ReadCharm
	ReadDir -> ReadCharmDir
	ReadBundle -> ReadCharmArchive
	ReadBundleBytes -> ReadCharmArchiveBytes

This prepares for adding the following new types (in a subsequent PR):

	Bundle
	BundleDir
	BundleArchive
	ReadBundleDir
	ReadBundleArchive
	ReadBundleArchiveBytes