Use user width when writing long string value label records in SAV writer

[gorcha]

The SAV writer functions that emit value labels for long string variables assume that the variable storage width is wider than the value label keys, but this is not checked during file writing so ReadStat can produce invalid files that don't throw an error on write (see tidyverse/haven#537).

This PR initially added a check for this during writing, including a minor update to the test suite to support tests with a manually specified storage width. While I was testing I noticed that PSPP isn't happy with using the storage width if it is wider than the user width and you get an error like:

warning: `test-550.sav' near offset 0x18c: Ignoring long string value label record for variable a because the record's width (16) does not match the variable's width (10).

I've updated the long string value writer to use the user width instead of storage width, and confirmed files now load successfully in PSPP (and still roundtrip successfully in ReadStat).

I think this fixes #323, tidyverse/haven#550 and Roche/pyreadstat#264.

[evanmiller]

Thanks!