psi_expr: Add psi_addsubexpr function

Append a subexpression to a compound expression.

(+ 1 2): compound expression
+, 1, and 2: subexpressions

((+ 1 2)): compound expression
(+ 1 2), +, 1, 2: subexpressions

In the last example +, 1, and 2 are all subexpressions, but the compound
they form, (+ 1 2), is itself a subexpression. These distinctions are
stored in the tree for the entire expression, so the behavior can
be recreated by traversing and evaluating the tree and its various
nodes.