getRUN limitation: must be paid in full before being extended

[dckc]

Describe the bug

In work such as #4092 that uses the so called attestation contract, the lien must be paid in full before it can be increased. This is an awkward limitation: a borrower might not have sufficient RUN on hand, but they may have enough BLD to back a larger loan. For example:

				Collateralization Ratio	RUN Borrowed		Liened BLD	RUN Rewards Accumulation	RUN Purse
Test #	Description	User Action Required	RUN per BLD	Before	After	Before	Delta	After	Staked BLD
3	Extending - more BLD required	100,600 -> 1600,10000	0.25	500%		100	1500	1600	10000

blocked by:

• Support Semi-Fungible Amounts with a CopyBag asset-kind #4361

cc @dtribble @Chris-Hibbert

[dckc]

@dtribble tells me he and @erights see a path to "semi-fungible" tokens, i.e. amounts where set items are paired with a Nat (iow: a bag).

@erights I look forward an issue about this. (p.s. #4361 is a relevant PR)

also, from Jan 4 discussion: One option that seems workable: Add “previous liened amount” to attestation amount. (and fold attestation into getRUN contract, find a new name, but keep it async)

[dckc]

@JimLarson notes some relevant tests / logic in some related go work; #3991 , I'm pretty sure.

[dckc]

Given #4361, this limitation went away during work on #3788.