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

tests/ui/needless_collect.rs

@@ -0,0 +1,10 @@
+#[warn(clippy, needless_collect)]
+#[allow(unused_variables, iter_cloned_collect)]
+fn main() {
+    let sample = [1; 5];
+    let len = sample.iter().collect::<Vec<_>>().len();
+    if sample.iter().collect::<Vec<_>>().is_empty() {
+        // Empty
+    }
+    sample.iter().cloned().collect::<Vec<_>>().contains(&1);
+}

tests/ui/needless_collect.stderr

@@ -0,0 +1,22 @@
+error: you are collecting an iterator to check its length
+ --> $DIR/needless_collect.rs:5:15
+  |
+5 |     let len = sample.iter().collect::<Vec<_>>().len();
+  |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider replacing with: `sample.iter().count()`
+  |
+  = note: `-D needless-collect` implied by `-D warnings`
+
+error: you are collecting an iterator to check if it is empty
+ --> $DIR/needless_collect.rs:6:8
+  |
+6 |     if sample.iter().collect::<Vec<_>>().is_empty() {
+  |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider replacing with: `sample.iter().any(|_| true)`
+
+error: you are collecting an iterator to check if contains an element
+ --> $DIR/needless_collect.rs:9:5
+  |
+9 |     sample.iter().cloned().collect::<Vec<_>>().contains(&1);
+  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider replacing with: `sample.iter().cloned().any(|&x| x == 1)`
+
+error: aborting due to 3 previous errors
+