[Bf-taskforce25] Editmesh accessor functions

[joe]

For bmesh development (to have an easier migration from editmesh) it'd
be nice if certain tools that use editmesh could still work even after
bmesh replaces editmesh.  Given the nonglobal design of 2.5, this
shouldn't be hard; simply have uses of editmesh go through accessors
functions.  These accessor functions would handle the necessary
conversions for editmesh tools to work with a bmesh.

It seems to me that the use of such accessor functions should be done
in the 2.5 branch, then merged over to bmesh, instead of the other way
around.  The 2.5 versions would obviously be fairly simple, just give
access to Mesh->edit_mesh; while the ones in bmesh would do the more
complex conversion necassary for editmesh operators to work.

Anyway, what does everyone else think?

Joe