Make public and external methods virtual to facilitate creation of custom SuperToken logic

[d10r]

Customized versions of the SuperToken logic currently cannot just inherit SuperToken.sol, because it doesn't declare its methods as virtual.

Examples:

• In Bridged supertoken custom-supertokens#26, updateCode changes.
• In https://github.com/GoodDollar/GoodProtocol/blob/master/contracts/token/superfluid/SuperToken.sol, the implementation of several methods (e.g. transfer) is changed.

In this PR, all public and external methods of SuperToken (and SuperfluidToken which it inherits from) are declared as virtual in order to facilitate creation of such customizations.

Additionally a few internal methods (for mint, burn, transfer, upgrade and downgrade) were also made virtual in order to allow changing the behaviour of that operations with less additional code.

[github-actions]

Changelog Reminder

Reminder to update the CHANGELOG.md for any of the modified packages in this PR.

• CHANGELOG.md modified
• Double check before merge

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +8.81% 🎉

Comparison is base (dae8ea5) 87.53% compared to head (688f7d1) 96.35%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1556      +/-   ##
==========================================
+ Coverage   87.53%   96.35%   +8.81%     
==========================================
  Files          91       41      -50     
  Lines        4389     2139    -2250     
  Branches      184        0     -184     
==========================================
- Hits         3842     2061    -1781     
+ Misses        503       78     -425     
+ Partials       44        0      -44     

Flag	Coverage Δ
ethereum-contracts	96.35% <0.00%> (ø)
sdk-core	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
...reum-contracts/contracts/superfluid/SuperToken.sol	98.58% <0.00%> (ø)
...contracts/contracts/superfluid/SuperfluidToken.sol	98.76% <ø> (ø)

... and 50 files with indirect coverage changes

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[hellwolf]

Code organization principles:

• _foo internal function as building blocks for reusability.
  • they should not be altered by subclasses.
  • "private" scope is generally avoided for simplicity, since test-harness may benefit from accessing them directly by subclassing it. This is often the case for our test cases.
  • The building blocks become "monadic", if it calls other abstract virtual functions. The naming convention for these functions are "_doSomeDamage".
• external functions can be created through assembling these internal logic together.
  • public functions is a shortcut used in code base to combine the definition of _foo and foo. This style should be gradually rid of from this code base, in favor of separation of definitions from day 0.

There are generally two ways of code logic reassembling:

function foo() external virtual override {
   // inject some logic before...
   super.foo();
   // inject some logic after...
}

or a complete rewrite of foo using _foo etc.

[hellwolf]

I still find it a bit stinky to be able to override so many things, but it is at least better than being able to override internal logic blocks.

[github-actions]

XKCD Comic Relif

Link: https://xkcd.com/1556