Should encodePassable format "compactOrdered" not use "~" as an encoding prefix?

[gibson042]

Describe the bug

As implied at

endo/packages/marshal/src/encodePassable.js

Lines 861 to 865 in b3a31b1

	* In addition, `|` is the remotable->ordinal mapping prefix:
	* This is not used in covers but it is
	* reserved from the same set of strings. Note that the prefix is > any
	* prefix used by any cover so that ordinal mapping keys are always outside
	* the range of valid collection entry keys.

and noted explicitly in Agoric/agoric-sdk#2589 (comment) , agoric-sdk liveslots expects | to be a prefix that sorts after all encoded values. But that assumption would no longer hold if ~-prefixed compactOrdered encodings were stored as-is. Here in endo, we can either assume that agoric-sdk will find a suitable workaround, or switch to a different prefix that does not suffer from this issue. The initial choice of ~ was mostly arbitrary, but did strive to avoid overloading any existing special character. Were we to stick with that goal, we'd be looking at candidates like

• U+0020 SPACE (which sorts before any other encoding but suffers from invisibility)
• { (which sorts after any other encoding but overloads the JSON begin-object token)
• " (which overloads the JSON string quote)
• # (which overloads the CapData body smallcaps prefix and smallcaps special-constant prefix)
• $/%/&/+/- (which overload smallcaps type prefixes)
• '/)/*/, (which are reserved by smallcaps but not currently used)
• .///;/</=/>/@/\/]/_/` (no known conflicts other than sorting in between legacy passable encodings)