Summary:
Update the `logging::objectToString()` and `appendToString)` functions when
converting objects that do not have a `toAppend(std::string*, Object)`
implementation.

Previously the code simply emitted `<no_string_conversion>` after the object
type name.  Now the code emits a hexdump of the object.  The emitted hexdump
size is based compile-type visible type of the object.  For subclass instances
the correct dynamic type name will be emitted (if RTTI is supported), but the
hexdump will only include the portion of object known from the compile-time
type.

Reviewed By: yfeldblum, chadaustin

Differential Revision: D14545305

fbshipit-source-id: d1e3df001b7f7c40c7bcb0888e76bf775512e5d3