Cache symbolic filesystem when serialising the engine

[Boyan-MILANOV]

Currently when serialising FileSystem, we dump the entire symbolic filesystem. When the filesystem contains many shared library files it results in a lot of overhead when dumping/loading states.

Since many shared library files will likely only be read by the program, and never modified, we could gain a lot of time and storage by caching their contents in memory and avoid serialising them when possible.

The best solution will probably to implement a dynamic cache for env files that keeps track of the PhysicalFile internal buffers with a copy-on-write mechanism.