Unify the `string`/`ipld.Node` information a parent has about its children when
updating a modified entry. This will be used to simplify the code around the
`childCloser` interface and related logic.

Use newly introduced `child` structure whenever the logically tied
`string`/`ipld.Node` pair was used to represent a child entry in a
directory operation.

Rename the `childCloser` interface to `mutableParent` to better reflect the fact
that the structures that implement it (`Directory` and `Root`) have the function
of parents in the MFS hierarchy. Rename also the method of the interface from
`closeChild` to `updateChildEntry` shifting the focus away from in which
circumstance is the method being called (when closing a child) to actually what
operation is being performed by the method (updating an entry in the parent to
the new content).

Extract the `Republisher` structure and related logic from `system.go` (leaving
that file mainly with the `Root` logic, and hence now renamed as `root.go`) to a
new `repub.go` file. Even though the MFS root has support for republishing
updates the logic behind those two structures is mostly decoupled, that should
be reflected in the source code organization.

Remove the `Sync()` method from the `FileDescriptor` and also its
implementation.

Two different undocumented methods in `FileDescriptor` are offered to update the
state of the MFS: `Flush()` and `Sync()`, both calling `updateChildEntry`. The
only difference (the user is not aware of) is that one sets the `fullSync`
argument and the other doesn't, that is, one does a full update of the
filesystem and the other just updates the parent directory.

This current situation is not clear to the consumer who may use one desiring the
effect of the other. As a precautionary measure while redesigning the MFS API
one method is removed to simplify its usage. The `Sync()` was chosen as the less
"safe" alternative which didn't do a full update of the MFS and also doesn't
seem to be used in the `go-ipfs` repository (the main consumer of the `mfs`
package).

Unify the two caches in `Directory` that discriminated between files and
directories (`files` and `childDirs`) into a single `entriesCache` since that
distinction wasn't used anywhere and made the code unnecessarily more complex.

Document the `Republisher` code in `repub.go` according to the explanation in
ipfs/kubo#5092 (comment). Erring on
the side of verbosity since this has been a part of the code with very little
review up until now.

Unify all the `Republisher` receivers names to `rp` (avoiding `r` to distinguish
it from the `Root` receivers).

The name `mutableParent` signals "this is a mutable parent, there are immutable
versions".