vm: replace vm-region with address-space

address-space is a non-recursive data structure, which contains a flat list of vm_areas representing
mapped vm-objects.

userspace programs can no longer create sub-address-spaces. instead, they can reserve portions of
the address space, and use that reserved space to create mappings.

include/kernel/channel.h

@@ -37,7 +37,7 @@ extern kern_status_t channel_read_msg(
 	struct channel *channel,
 	msgid_t msg,
 	size_t offset,
-	struct vm_region *dest_region,
+	struct address_space *dest_region,
 	const kern_iovec_t *dest_iov,
 	size_t dest_iov_count,
 	size_t *nr_read);
@@ -45,7 +45,7 @@ extern kern_status_t channel_write_msg(
 	struct channel *channel,
 	msgid_t msg,
 	size_t offset,
-	struct vm_region *src_region,
+	struct address_space *src_region,
 	const kern_iovec_t *src_iov,
 	size_t src_iov_count,
 	size_t *nr_written);