Process the body of a backquote.
S is the body. Returns a cons cell whose cdr is piece of code which
is the macro-expansion of S, and whose car is a small integer whose value
can either indicate that the code is constant (0), or not (1), or returns
a list which should be spliced into its environment (2).
LEVEL is only used internally and indicates the nesting level:
0 (the default) is for the toplevel nested inside a single backquote.