Make the combiner macros less likely to cause name collisions.

Make the combiner macros less likely to cause name collisions.

Protect the arguments to the combiner macros with parentheses, and
postfix their temporary variables with underscores to avoid name space
collisions with the surrounding code.

Alexander Shulgin pointed out that underscore-prefixed identifiers are
reserved for the C implementation, so we use postfix underscores
instead.