Use short type suffix for cache files

Use short type suffix for cache files

x.result -> xR
x.manifest -> xM
x_n.raw -> xnW

Reducing file lengths should be beneficial since it reduces the number
of needed system calls when scanning many files in the cache. The effect
is very small but there is no real downside.

A one letter suffix is kept to allow for making decisions based on file
type without having to open the file. The suffix is uppercase so that is
doesn’t clash with pre-4.0 *.stderr files stored in the cache.