feat: add new_with_capacity to Assembler

[nhtyy]

I think most use cases do have a lower bound on the number of emitted instructions, and using VecAssembler requires extra steps to make the code executable.

[CensoredUsername]

Hi! Thank you for the contribution. I understand the need for such an API. However, I have some comments.

First, this API would now take a certain amount of instructions as an argument. I would recommend just taking the size as bytes. You've now assumed an instruction size of 7 bytes, which could be correct for x64/x86, but would be wildly incorrect for the riscv/aarch64 backends. This is a detail is probably best exposed to the programmer to begin with, rather than assuming a certain average instruction size.

Second, I would strongly recommend however that MemoryManager is always initialized with a multiple of the memory page size. Memory permissions can only be changed on a page size basis for most platforms, so reserving anything less than a full page doesn't actually save anything. R::page_size() contains the size of a memory page in bytes that you need.

[nhtyy]

Pushed a commit that should fix your concerns :)

[CensoredUsername]

Thanks! I'll cut a release with this in a week or so.