prefix for public symbols of Fortran bindings

The module memman currently provides the Fortran bindings in an inconsistent way: Some public symbols are prefixed with mm_ others are not.

The arguments in favour of having no prefix:

shorter names / less boilerplate
to avoid collisions the user can always alias the names within the USE statement

The arguments in favour of a prefix:

it is easier to identify the Mmgr calls within ICON
renaming within the USE statement is not convenient and adds non-local complexity to the application

The task of this issue is to resolve the discussion and to initiate a MR to implement the solution.

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