Lint against needless uses of collect()

Handles cases of `.collect().len()`, `.collect().is_empty()`, and
`.collect().contains()`. This lint is intended to be generic enough to
be added to at a later time with other similar patterns that could be
optimized.

Closes #3034

clippy_lints/src/lib.rs

@@ -554,6 +554,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         loops::ITER_NEXT_LOOP,
         loops::MANUAL_MEMCPY,
         loops::MUT_RANGE_BOUND,
+        loops::NEEDLESS_COLLECT,
         loops::NEEDLESS_RANGE_LOOP,
         loops::NEVER_LOOP,
         loops::REVERSE_RANGE_LOOP,
@@ -904,6 +905,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         escape::BOXED_LOCAL,
         large_enum_variant::LARGE_ENUM_VARIANT,
         loops::MANUAL_MEMCPY,
+        loops::NEEDLESS_COLLECT,
         loops::UNUSED_COLLECT,
         methods::EXPECT_FUN_CALL,
         methods::ITER_NTH,