Improve stave access by caching index

[miiizen]

This PR attempts to cut down the time spent in Staff::idx (called 3,745,816 times in the Beethoven) and related Score staff functions. Previously, the index of a Staff in a score was determined by it's location in the Score's m_staves vector. Finding the index when given a Staff* required using std::find.

We also need to be able to find a Staff* given a staff_idx_t. This is used even more frequently (101,357,900 times in the Beethoven!) I have changed this to use [idx] rather than .at(idx), as we do our own bounds checking before this call. This appears to have increased performance in this function by 1.2% but I think this is firmly in the bounds of error.

To maintain direct access to the Staff* pointer by index and allow direct access to the index by Staff* pointer, I have simply added a staff_idx_t member to Staff. This is straightforward to keep track of as there are only 3 methods which modify the m_staves variable - Score::appendStaff, Score::insertStaff and Score::removeStaff.

Overall, the improvements are slim. There is no significant change in total score layout, but Staff::idx has been sped up by 21%.