Added the following statistics (per engine) to FUNCTION STATS command:
* number of functions
* number of libraries
Output example:
```
> FUNCTION stats
1) "running_script"
2) (nil)
3) "engines"
4) 1) "LUA"
2) 1) "libraries_count"
2) (integer) 1
3) "functions_count"
4) (integer) 1
```
To collect the stats, added a new dictionary to libraries_ctx that contains
for each engine, the engine statistics representing the current libraries_ctx.
Update the stats on:
1. Link library to libraries_ctx
2. Unlink library from libraries_ctx
3. Flushing libraries_ctx
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