part/persist: code refactoring

move subroutines from header to source files.

Many subroutines are only used by a single source file,
so move them out of the header file. This will make it
easier for debuggers since setting a breakpoint in a header file
is not always correctly supported.

Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>

Author: ggouaillardet