Fix compound indexes for dynamic models

[xquezme]

Right now if you try to create any compound index for core store dynamic model you will receive syntax error. This because "-" is prohibited and caused syntax error. For instance, for Dog model in DynamicModelTests.swift you will receive this error:

near "-": syntax error in "CREATE INDEX IF NOT EXISTS Z_Dog__CoreStoreSchema_indexes_Dog_age-nickname ON ZANIMAL (ZAGE COLLATE BINARY ASC, ZNICKNAME COLLATE BINARY ASC)

Let's use "_" instead.

[xquezme]

@JohnEstropia Can you take a look, please?

[JohnEstropia]

@xquezme Thanks for this PR and apologies for not responding here sooner, but I did check your PR and acknowledge the issue.
I've been meaning to look for a formal fix for this, but haven't found time yet to do a full test on the behavior.

The reason I can't merged the PR as it is now is because unfortunately, the current NSFetchIndexDescription.name actually works for single-indexes. By changing the naming now, it might break existing apps that use the current naming. I'll get to a formal fix and unit tests for it once I release the Swift 5.5/Xcode 13 update, but if you find a solution ahead feel free to update the PR.