Deparser: Set A_EXPR node context for deparseExpr and deparseFuncCall

This allows us to handle keyword-style FuncCalls correctly in
deparseFuncCall, as will be done in a subsequent commit.

Note that the safe choice here is to set the context to NONE, which
intends to add parenthesis in places where a_expr is not allowed
(b_expr or c_expr can be turned into an a_expr by being wrapped in parens)