Re-using Session as a variable name risks confusion with the Session
type.

This argument serves as an opaque id (outside the ControllerAccess
object) for a call to a wrapper function. I expect that most
ControllerAccess implementations will want to use this argument as a
sequence number (plain integer), for which uint64_t will be a better fit
than void*. For ControllerAccess implementations that want to use a
pointer, uint64_t should be sufficiently large.

This replaces SPS transparent conversion for pointers. Transparent
conversion only applies to argument/return types, not nested types. We
want to be able to serialize / deserialize structs containing pointers.

We may need to replace this in the near future with a new SPSPointer tag
type, since SPSExecutorAddr is meant to be serialization for pure
addresses, and pointers may carry other information (e.g. tag bits), but
we can do that in a follow-up commit.

SPS deserialization for span<const char> produces a value that points
directly into the argument buffer for efficiency. This was broken by an
unnecessary std::move of the buffer inside WrapperFunction, which this
commit removes.

Ensures that SPS transparent conversion will apply to arguments passed
by reference.

Related tests use "TransparentConversion" rather than
"TransparentSerialization".

The DirectCaller utility allows "direct" calls (with arguments
serialized into, and then immediately back out of a
WrapperFunctionBuffer) to wrapper functions. It was introduced for the
SPSWrapperFunction tests, but will be useful for testing WrapperFunction
interfaces for various orc-rt APIs too, so this commit hoists it
somewhere where it can be reused.

Expected<T*> values will be converted to/from Expected<ExecutorAddr>
values.

Drop the redundant 'Test' prefix and rename transparent serialization tests to
clarify their purpose.

Allows SPS wrapper function calls and handles to use pointer arguments.
These will be converted to ExecutorAddr for serialization /
deserialization.