Inconsistent asserts and default annotation parameters

[dtebbs]

Various places in the code (e.g. pb_variable_array) provide a default annotation_prefix parameter of "", but then assert if this is passed in. e.g. https://github.com/clearmatics/libsnark/blob/master/libsnark/gadgetlib1/pb_variable.tcc#L29.

[dtebbs]

At the same time as enforcing annotations everywhere, it makes sense to determine exactly what the DEBUG build flag does (AFAIK it makes further use of the annotations), and potentially factor out DEBUG to one or more LIBSNARK_... build variables which enable the extra functionality. (DEBUG is pretty ambiguous for this kind of thing).

[dtebbs]

Note that FMT (which is commonly used to create the annotation strings) is also controlled by DEBUG (i.e. is DEBUG is not set, FMT expands to a no-op, returning an empty string. Thus, in the current code, the check for non-empty strings can only be enabled when DEBUG is set, and FMT works as expected.