Refactor Process struct

[frisitano]

As part of #1082, the introduction of the Host object and associated ProcessState, we had to place the Host object behind a RefCell to leverage the interior mutability pattern and defer borrow checks to runtime. This is because we take a mutable and immutable reference of the Process object when invoking Host requests. However, we never modify any of the fields of the Process struct which are read as immutable. To remove the requirement for the usage of the RefCell we should refactor the Process struct as follows:

struct Process<H>
where
    H: Host,
{
    state: VmState,
    host: H,
    max_cycles: u32,
}

See this comment thread for more insight: #1082 (comment)

[bobbinth]

Superseded by changes in #1571.