buflib: add pin/unpin operation

An allocation is pinned by calling buflib_pin() to up its pin count.
The pin count is like a reference count: when above 0, buflib won't
move the allocation and won't call its move callbacks. This makes it
safe to hold the pointer returned by buflib_get_data() across yields
or allocations.

Note that pinned allocations can still shrink because there are some
use cases where this would be valid, if buffer users coordinate with
the shrink callback.

Change-Id: I0d0c2a8ac7d891d3ad6b3d0eb80c5b5a1b4b9a9d

firmware/core_alloc.c

@@ -104,6 +104,21 @@ bool core_shrink(int handle, void* new_start, size_t new_size)
     return buflib_shrink(&core_ctx, handle, new_start, new_size);
 }
 
+void core_pin(int handle)
+{
+    buflib_pin(&core_ctx, handle);
+}
+
+void core_unpin(int handle)
+{
+    buflib_unpin(&core_ctx, handle);
+}
+
+unsigned core_pin_count(int handle)
+{
+    return buflib_pin_count(&core_ctx, handle);
+}
+
 const char* core_get_name(int handle)
 {
     const char *name = buflib_get_name(&core_ctx, handle);