[LFSC] Fix memory leaks when creating CExprs

[LFSC] Fix memory leaks when creating CExprs

In certain cases, LFSC was creating CExprs with the single-argument
constructor, which allocates an array of one child, only to immediately
replace it with a new array (without deleting the old one).
Additionally, this commit fixes the construction of TYPE/KIND/MPZ/MPQ
expressions (the null pointer is appended automatically by the single
argument constructor, an array with two null pointer entries should not
be necessary).