Refactor low-level functions from bmap command line tool into some library functions

[Razaloc]

Right now the target file is truncated before going through the copy function. So bmap::copy should absorb that function and sort it. The main file does not need to carry low level functions, should focus on sorting the commands.

[sjoerdsimons]

Actually no; The main bmap::copy should stay as generic as possible; Potentially the library crate could get some extra helper functions to deal with different/common outputs but that shouldn't be part of the copy function

[obbardc]

right, bmap::copy is fine how it is: what this task should describe is refactoring some of the file processing out of the bmap command line tool into a library such that it could be better used in tests, or other scripts.