WIP feat: improve grc20/foo20 APIs

[moul]

Context: from a live discussion when reviewing #938.

Goals:

• better names
• better API (around the banker)
• support for "Local banker" for realms

Checklists...

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• Added new tests, or not needed, or not feasible
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• Checked that the author followed the guidelines in CONTRIBUTING.md
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[albttx]

NOTE: Banker20 should have a caller variable (set to nil by default) for example which when you call GRC20() that return a IGRC20. force to use the caller variable.

This fix when you call from a realm the external method, or the internal method when you get the struct

myfoo20 := foo20.SafeGRC20()
myfoo20.Transfer() // <-- std.PrevRealm could lead to error