Add ability to set Limits per Instance

[tyler]

Currently a Region is created with a set of Limits. Those Limits are applied to the Region (in order to correctly lay out memory) as well as to the Instances in how they behave at runtime, how much memory they are allowed to allocate and so on.

We would like to be able to have Limits set per-Instance.

That has some small complications. For instance, the Limits for the instance must generally be "smaller" than the Limits set on the Region in order for it to be safe and not turn into a crazy bin-packing problem. This will likely also require making where certain things are allocated for the instance within the Slot dynamic. Probably other little issues as well.

[fst-crenshaw]

TL;DR: When instances are created in lucent-runtime, add a Limits parameter, so that different instances may be executed with different runtime limits.

Within the lucent-runtime, a Region is the memory region from which lucet instances are created and run. A Region has a collection of Slots from which instances are created, and when a particular slot is live, it also has an associated Alloc.

The three traits -- Region, RegionInternal, and RegionCreate -- when implemented by a type, offer the functionality necessary to create a new region, create a new instance from the region, assess the capacity of a region, and drop an instance from the region. Currently, the RegionCreate trait creates a region with a fixed capacity and fixed runtime limits on things like heap size, stack size, and global size.

According to the RegionInternal trait, when a new instance is created with new_instance_with(), one must supply a module and an embedded context. In the Mmap implementation of new_instance_with, we see that the limits imposed on a Region are simply copied into the Slot. Here:

lucet/lucet-runtime/lucet-runtime-internals/src/region/mmap.rs

Lines 96 to 102 in 41c2daf

	for (ptr, len) in [
	// make the stack read/writable
	(slot.stack, limits.stack_size),
	// make the globals read/writable
	(slot.globals, limits.globals_size),
	// make the sigstack read/writable
	(slot.sigstack, limits.signal_stack_size),

Enhance the trait new_instrance_with() so that one may pass in a heap_memory_size parameter. Enhance the mmap implementation of Region so that it implements this new limiting feature. Enforce the condition that a slot's 'heap_memory_size` cannot violate 4kb boundaries or be greater than the limits supplied to the region.