Need to implement may_enter and may_leave flags

[alexcrichton]

These have since been added to the spec and it'll ideally be more obvious where to implement these once bytecodealliance/wit-bindgen#314 is done. Once implemented this should also lead to the implementation of "lockdown semantics" where if a component traps it prevents the entire component from being reentered.

[guybedford]

Now that instance flags are supported, the groundwork for this check is all in.

What remains is to implement setting and checking the flags, defined as:

pub const FLAG_MAY_LEAVE: i32 = 1 << 0;
pub const FLAG_MAY_ENTER: i32 = 1 << 1;

where the flags are set and checked per instance, verified per the canonical ABI within the correct function call positions.