Allow specifying custom hashing functions

[Mrtenz]

For performance reasons, it may be beneficial to use custom hashing functions (i.e., pbkdf2, hmac) instead of the ones used in key-tree by default. Especially in React Native, the native function seems to be much quicker.

[FrederikBolding]

@Mrtenz It may be worth it to explore using globalThis.crypto where possible too as well, since there's a high chance that this will be faster inside the mobile execution environment as well.