Add oZFile_fwrite method

[ahl27]

Adds analogous fwrite method for oZFile accessors. fwrite tends to be slightly faster than puts since it doesn't have to check every character for '\0', which allows for some OS optimization. Would be useful to use in Biostrings for writeXStringSet since the write size of blocks are known in advance, so appending '\0' and then asking the OS to check for it is overkill.

Function arguments follow conventions of the other oZFile* functions despite differing from fwrite argument syntax.

gzfwrite is defined here:

https://github.com/madler/zlib/blob/ef24c4c7502169f016dcd2a26923dbaf3216748c/gzwrite.c#L260-L262

Header definition is here:

https://github.com/madler/zlib/blob/ef24c4c7502169f016dcd2a26923dbaf3216748c/zlib.h#L1472-L1473

I moved the n==nitems check to the end of the switch case because I find it significantly more readable than the alternative in oZFile_puts.

[ahl27]

force pushes are some minor commit amends