Pre-allocate memory in extend_from_slice, extend and append functions (#80)

* change behavior on extend_from_slice, append

* preallocation in extend()

* append v2

* obviously

* fix resize_with

tests/tinyvec.rs

@@ -99,3 +99,4 @@ fn TinyVec_macro_non_copy() {
   let s = String::new();
   let _: TinyVec<[String; 10]> = tiny_vec!([String; 10] => s);
 }
+