This PR has some very minor substantive changes with accompanying tests and several small optimizations for kern import

Substantive changes:

1. Grace notes without slashes are now detected when two lowercase q's are present
2. Grace notes apply their stated duration rather than always having eighth note representations. About these two grace note points, you can see in this example that humdrum is sensitive to the stated duration of a grace note, and also that the second q will omit the slash.
3. In chords where only the first note has a stated duration, the other notes of the chord will take on the duration of the first note. The same exact duration object is used, similar to how chords already currently reuse the duration of their first note. Although standard humdrum syntax calls for a duration on each note of a chord, you can see here that when only the first note has a stated duration, other chords will use the same rather than defaulting to a quarter note.

Kern import optimizations (mostly in hdStringToNote)

1. if str.count('x'): becomes if 'x' in str:
2. Move duration parsing of notes to the beginning of hdStringToNote so that notes and rests can be instantiated with their correct duration from the beginning rather than being created and then having their duration replaced. This prevents an unused default duration from being created on each note and rest. Even better for chords, this pr makes them inherit the duration of their first note when the chord is created rather than right after creation, which means that instead of creating 1 unused duration per chord, now no new ones are created. I didn't check, but this super simple change for chords could be a quick and easy win for other import types if they construct chords in the same way.
3. Combine the normal duration and rational duration regexes into one. Likewise for note name and accidental searches.
4. Use walrus operator where possible to avoid duplicating operations
5. Add short-circuit to stop processing a note/rest if all the characters have been accounted for after the duration and note name/accidental have been processed
6. Comment out parsing that only results in pass statements
7. Memoize the duration and note name regex searches inside of hdStringToNote. New durations, notes, and rests are still instantiated for each event.