Fix shadowing warning

[minaminao]

The following warning is displayed when compiling. This PR will modify so that the warning is not displayed. See also https://github.com/ethereum/solidity/pull/12680 for more information on this style.

Warning: This declaration shadows an existing declaration.
  --> src/strings.sol:45:42:
   |
45 |     function memcpy(uint dest, uint src, uint len) private pure {
   |                                          ^^^^^^^^
Note: The shadowed declaration is here:
  --> src/strings.sol:85:5:
   |
85 |     function len(bytes32 self) internal pure returns (uint) {
   |     ^ (Relevant source part starts here and spans across multiple lines).
Note: The shadowed declaration is here:
   --> src/strings.sol:160:5:
    |
160 |     function len(slice memory self) internal pure returns (uint l) {
    |     ^ (Relevant source part starts here and spans across multiple lines).

[refcell]

This would be much appreciated!

[radeksvarz]

@RyanRHall what's holding from applying this small PR? Is there any help needed?

[Sabnock01]

Would like to see this merged. Will open my own PR for it if necessary.

[minaminao]

Hi @Arachnid. Thanks for a great library. This library is used by many projects, and some people find it a little annoying that warnings are shown when compiling them. This PR just changes len to length. I'd appreciate it if you could merge this PR when you have time :)

[Arachnid]

Thanks! And sorry for the long delay.