Spread transfer request over multiple accounts matching a selector

[SOF3]

For example, if we have a database like this:

ID	Balance	Labels
X01	15	name=X, valueMin=0
X02	15	name=X, valueMin=0
Y03	0	name=Y, valueMax=100

We want to transfer 20 from name=X to name=Y. This is not possible using a single account, but this is possible by splitting the transfer into multiple transactions:

Source	Destination	Delta
X01	Y03	15
X02	Y03	5

Alternatively:

Source	Destination	Delta
X01	Y03	10
X02	Y03	10

Whichever strategy we want to use, this is not possible with the current SQL procedures. To implement this with atomic transactions on MySQL, we may need to create a table storing a temporary list of accounts matching a label selector, then perform operations based on this temporary list.

[paxbun]

I have a similar issue due to this. If you think it's okay, I want to make a PR that adds enum variants Char8, Char16, Char32, and WChar, which correspond to the u8, u, U, and L prefixes.