flash loans

[0xripleys]

Port of #70 + some tweaks

Quick Summary: You can now flash borrow funds from a reserve in instruction i as long as you flash repay the funds + fees in instruction j where i < j.

Couple of things to note:

1. I deprecated the old flash loan instruction. I don't think there's an exploit, but better to be safe than sorry.
2. There can only be 1 flash borrow/repay per transaction. This constraint might be relaxed later.
3. the flash repay instruction needs to know the index of the flash borrow instruction. this is to prevent lone flash repays in a transaction
4. I am also considering adding the requirement flash_borrow.destination_liquidity_info == flash_repay.source_liquidity_info, for extra safety.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.80445% with 166 lines in your changes missing coverage. Please review.

Project coverage is 81.37%. Comparing base (1feaffc) to head (c108e99).
Report is 31 commits behind head on upcoming.

Files with missing lines	Patch %	Lines
token-lending/program/tests/flash_borrow_repay.rs	88.72%	91 Missing ⚠️
token-lending/program/src/processor.rs	78.59%	55 Missing ⚠️
...-lending/program/tests/helpers/flash_loan_proxy.rs	89.85%	14 Missing ⚠️
token-lending/program/src/instruction.rs	90.19%	5 Missing ⚠️
token-lending/program/src/state/reserve.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           upcoming      #95      +/-   ##
============================================
- Coverage     81.85%   81.37%   -0.48%     
============================================
  Files            32       33       +1     
  Lines          7681     8633     +952     
============================================
+ Hits           6287     7025     +738     
- Misses         1394     1608     +214     

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[nope-finance]

i guess will change other place to do like this too