Add function + tests to convert decimals with token

[joYyHack]

• Since this PR suggests a bug fix, the tests have been added and the coverage is 100%.
• Since this PR introduces a new feature, the update has been discussed in an Issue or with the team.
• This PR is just a minor change, like a typo fix.

---

Added token decimals converter. The function accepts:

1. The amount of the base token.
2. The base token address.
3. The destination token address.

Under the hood, the function casts the addresses of the tokens to the IERC20Metadata interface and retrieves the decimals for both tokens. Then, it converts the amount with the decimals of the base token to the decimals of the destination token.

All necessary tests are added. Coverage 100%.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (0b4265c) 100.00% compared to head (49627d3) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #61   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           35        35           
  Lines         1280      1301   +21     
  Branches       156       157    +1     
=========================================
+ Hits          1280      1301   +21     

Files	Coverage Δ
contracts/libs/decimals/DecimalsConverter.sol	100.00% <100.00%> (ø)

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[Hrom131]

Let's add functions to18 and from18, which will take the address of the token, instead of decimals

[Arvolear]

Let's add functions to18 and from18, which will take the address of the token, instead of decimals

This is indeed what will be useful

[joYyHack]

Let's add functions to18 and from18, which will take the address of the token, instead of decimals

This is indeed what will be useful

Okay, that's a good idea, but I need to warn you that it appears impossible to overload 'safe' functions because they all utilize the 'convert' function, which accepts a function as a parameter. So, for example, if we have both
'to18(uint256, uint256)' and 'to18(uint256, address)' along with 'convert(function(uint256, uint256)),' and we attempt to call 'function test { convert(to18) },' the compiler will report an error, stating 'No matching declaration found after variable lookup.' It seems that the compiler can't determine which of the overloaded functions we're trying to use.

Overall the idea is that I will just name it differently :D

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