Add Mutation as a trait

[aejenk]

Mutation should be added as a trait, with a mutate function. This function should take two parameters - MmapMut, and a Vector of MutOption.

MutOption is an enum specifying options of how the mutation should be done. For example, ChunkSize(u64) would be an option specifying how large each chunk to mutate is. Mutations may use the required MutOptions and ignore the rest.

A helper function process_options can be made to retrieve the relevant options and return them in a struct, wrapped around a Result. Ok would contain the parsed options, and Err would mean that required options are not present. This is optional however.

[aejenk]

Update explanation: In MmapMut you can use get_mut to retrieve a slice of mutable bytes, however this would limit the capabilities of a Mutation.

A Mutation should mutate the file, not a chunk. The reasoning behind this is that some mutations can extend beyond the chunk. For example, if I have:

[1,2,3,4,5,6]

...and the slice is 1,2,3 - however the Mutation acts by, for example, adding this chunk to the next chunk, the expected value would be this:

[1,2,3,5,7,9]

Another option is that the Mutation might wrap around from the end of the file, back to the beginning. Hence, why the whole map needs to be passed.

An alternative to this is passing a mutable reference to the mapped file - but this should be tested for practicality. If it works, however, then it is preferred.

[aejenk]

Update to issue: the mutate function will still contain two arguments, the first one being the data, however the second one would become another type of custom enum - HashMap<&str, OptionVal>

OptionVal would be an enum with the following values:

enum OptionVal {
    OString(String)
    OInt(isize)
    OBool(bool)
    OArray(Vec<OptionVal>)
}

This way, custom objects can be implemented dynamically. Each String name would map onto an OptionVal. containing the values specified.

Thanks to @LunarLambda for this idea!