Closes#913.
Provide package info in the funcScope to check if function is defined
insided an interop package. As a good side-effect bytecode for builtins from `util`
is no longer emitted.
Related #941.
By default VM initializes every field of a new struct to a Boolean.
If field contains another struct, we need to initialize it to default
value explicitly. This commit sets default values for uninitialized
field.
1. Slot is a new mechanism for storing variables during execution
which is more convenient than alt.stack. This commit implements
support for slot opcodes in both vm and compiler.
2. Remove old alt.stack opcodes.
3. Do not process globals at the start of every function, but instead
load them single time at main.
Previously, struct variables were initialize with VM's nil value
which is of primitive type. Thus SETITEM used for struct's field
updating wasn't working.
Previously this declarations were ignored which resulted
in runtime errors, because VM's nil is an element of primitive type
and can't be converted to an array.
