Before we created an empty file and asked the OS to copy the file.
The OS is very good at this so this is a good idea generally. However
it seems that in some cases, possibly an Android Dowload folder, we
might be able to create a file but not overwrite it. Thus refactor
this a bit so we are copying the file ourselves.
There are no new tests here since the behaviour remains identical.
The good news is that the existing tests were good enough to catch
some bugs already.
This more strongly types the ubiquitous message id type by no longer
making it an integer. It keeps the actual ID opaque. Only for the
generic job API the number keeps being used. Some locations also need
to create it from an integer and call MsgId::new().
This creates a specific type for blobs, with well defined conversions
at the borders. It also introduces a strong type for the Param::File
value since that param is often used used by the public API to set
filenames using absolute paths, but then core changes the param to a
blob before it gets to the database.
This eliminates a few more functions with very mallable C-like
arguments behaviour which combine a number of operations in one.
Because blob filenames are stored so often in arbitrary strings this
does add more code when receiving those, until the storage is fixed.
File name sanitisation is now deletated to the sanitize-filename crate
which should do a slightly better job at this.
