Use mremap on Linux instead of munmap+mmap

[ajeetdsouza]

Linux exposes an mremap function that is not available on other OSes, that allows us to remap an address space, providing a more efficient way to increase the size of an mmap file. This PR allows Linux systems to use the mremap call, while other OSes continue to use munmap followed by mmap.

sys does not provide an mremap function, so this had to handwritten using a system call. Unfortunately, the functions provided by sys for Mmap/Munmap maintain a list of active mappings, and do not allow us to Munmap anything that is not present in the list. Since our handwritten mremap call cannot update that internal list, we must reimplement the munmap system call as well.

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This change is 

[jarifibrahim]

Clean it up and it's good to merge