perf: Look into optimizing memory allocations within apply_diff()

[neoncitylights]

Because String.insert() and String.remove() are both O(n) (linear) operations, it'd be useful to look into reducing the number of internal calls to both of these methods. There is one way I'm thinking of at the moment, which is to look for series of length-shifting operations.

We can keep track of the last diff operation, and compare it to the current one.

• Inserts: reduce a series of push calls into a single push_str() call
• Deletes: reduce a series of remove calls into a single remove() call

[neoncitylights]

@notalfredo wanna try this?

[notalfredo]

@notalfredo wanna try this?

sure I can try it out