These functions allow you to look up types given a name in a simple
subset of C declarator syntax (no function pointers), to look up the
types of variables given a name, and to look up the types of data
objects and the type signatures of functions given symbol table offsets.
(Despite its name, one function in this commit, ctf_lookup_symbol_name(),
is for the internal use of libctf only, and does not appear in any
public header files.)