Support anonymous named and anonymous shared shared mappings in /proc/PID/maps

Linux allows naming anonymous mappings, both shared and non-shared. Named anonymous mappings appear as `[anon:]` entries while anonymous shared named mappings appear as `[anon_shmem:]` lines (see the [kernel documentation](https://docs.kernel.org/filesystems/proc.html)). Since in both cases the name is optional I propose to extend the `MmapPath::Anonymous` with an optional string name, so `MmapPath::Anonymous(Option)` and introduced a similar entry for anonymous shared mappings (say `MmapPath::AnonymousShared(Option)`.