Separate API definition from implementation

Separate API definition from implementation

The new file `<side/api.h>' defines the DSL and expands to its
implementation in `<side/instrumentation-c-api.h>'.

Introduce the following DSL change: identifiers defined by
`side_{arg,}_define_{array,struct,vla,enum,variant,optional}'
must not be rereferenced with the `&' operator, but simply by their
name.

Change-Id: I883de215000e10abfbd27c93fddba3bae3abf5b2
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>