Prefix all symbols with lean4-mode

[mekeor]

The Elisp manual states:

Since all symbols are stored in a single obarray, programmers commonly prefix each symbol name with the name of the library where it originates. For example, the functions ‘text-property-search-forward’ and ‘text-property-search-backward’ both belong to the ‘text-property-search.el’ library. By properly prefixing symbol names, one effectively prevents clashes between similarly named symbols which belong to different libraries and thus do different things.

Thus, we should rename our Elisp-files and -symbols to use the lean4-mode prefix.