[kernel] First steps at removing channel objects

This commit does a number of things to start the transition of channels
from kernel to user space:

- Remove channel objects / syscalls from the kernel
- Add mutex type in libj6
- Add condition type in libj6
- Add a `ring` type flag for VMA syscalls to create ring buffers
- Implement a rudimentary shared memory channel using all of the above

definitions/objects/channel.def

@@ -1,21 +0,0 @@
-object channel : object {
-    uid 3ea38b96aa0e54c8
-
-    capabilities [
-        send
-        receive
-        close
-    ]
-
-    method create [constructor] 
-    method close [destructor cap:close]
-
-    method send [cap:send] {
-        param data buffer [inout]
-    }
-
-    method receive [cap:receive] {
-        param data buffer [out]
-        param flags uint64
-    }
-}

definitions/syscalls.def

@@ -1,6 +1,5 @@
 import "objects/object.def"
 
-import "objects/channel.def"
 import "objects/event.def"
 import "objects/mailbox.def"
 import "objects/process.def"
@@ -12,12 +11,12 @@ interface syscalls [syscall] {
     uid 01d9b6a948961097
 
     expose ref object
-    expose ref system
+
     expose ref event
-    expose ref process
-    expose ref thread
     expose ref mailbox
-    expose ref channel
+    expose ref process
+    expose ref system
+    expose ref thread
     expose ref vma
 
     # Simple no-op syscall for testing