st_file.pl -- File handling

Handles file paths and caching.

st_cached(+File, -Template) is semidet
Retrieves the cached template. Fails when no such template is cached.
st_cache_invalidate is det
Purges all cache entries.
st_resolve(+File, -AbsFile, +Options) is det
Resolves file name relative to current working directory. Adds extension.
st_resolve_include(+Include, +File, -Abs) is det
Resolves included file against the current file. Assumes that File is absolute path.
st_cache_put(File, Template) is det
Puts the template into cache. Does nothing when caching is not enabled.