Added TinyVec::with_initial_len() function and associated tests.

[lordmulder]

Makes a default-initialized TinyVec with the given initial length.

If the requested length is less than or equal to the array capacity you
get an inline vec. If it's greater than you get a heap vec.

let t = TinyVec::<[u8; 10]>::with_initial_len(5);
assert!(t.is_inline());
assert_eq!(t.len(), 5);

let t = TinyVec::<[u8; 10]>::with_initial_len(20);
assert!(t.is_heap());
assert_eq!(t.len(), 20);

This fixes #210.