Add lint for ifs that could be collapsed

"Collapsible" ifs are ones which contain only a then block, and the then
block consists of an if that only has a then block.

src/lib.rs

@@ -25,6 +25,7 @@ pub mod eta_reduction;
 pub mod identity_op;
 pub mod mut_mut;
 pub mod len_zero;
+pub mod collapsible_if;
 
 #[plugin_registrar]
 pub fn plugin_registrar(reg: &mut Registry) {
@@ -45,6 +46,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_lint_pass(box mut_mut::MutMut as LintPassObject);
     reg.register_lint_pass(box len_zero::LenZero as LintPassObject);
     reg.register_lint_pass(box misc::CmpOwned as LintPassObject);
+    reg.register_lint_pass(box collapsible_if::CollapsibleIf as LintPassObject);
     
     reg.register_lint_group("clippy", vec![types::BOX_VEC, types::LINKEDLIST,
                                            misc::SINGLE_MATCH, misc::STR_TO_STRING,
@@ -61,5 +63,6 @@ pub fn plugin_registrar(reg: &mut Registry) {
                                            mut_mut::MUT_MUT,
                                            len_zero::LEN_ZERO,
                                            len_zero::LEN_WITHOUT_IS_EMPTY,
+                                           collapsible_if::COLLAPSIBLE_IF,
                                            ]);
 }