linear memory via mmap

[frehberg]

linear memory is located in virtual memory o process.
growing the linear memory may require to re-alloc complete array and copying from old to new, larger one.

Instead mmap should be used to reserve the virtual linear memory, mapping it directly onto physical memory pages; this way linear could grow without re-alloc, but just mapping additional physical memory pages into the consecutive mmap-memory address space.

[NikVolf]

Hi, sure, this is the issue we have in wasmi interpreter!
wasmi-labs/wasmi#45