* winch(x64): Initial implementation for function calls
This change adds the main building blocks for calling locally defined
functions. Support for function imports will be added iteratively after this
change lands and once trampolines are supported.
To support function calls, this change introduces the following functionality to
the MacroAssembler:
* `pop` to pop the machine stack into a given register, which in the case of
this change, translates to the x64 pop instruction.
* `call` to a emit a call to locally defined functions.
* `address_from_sp` to construct memory addresses with the SP as a base.
* `free_stack` to emit the necessary instrunctions to claim stack space.
The heavy lifting of setting up and emitting the function call is done through
the implementation of `FnCall`.
* Fix spill behaviour in function calls and add more documentation
This commits adds a more detailed documentation to the `call.rs` module.
It also fixes a couple of bugs, mainly:
* The previous commit didn't account for memory addresses used as arguments for
the function call, any memory entry in the value stack used as a function
argument should be tracked and then used to claim that memory when the function
call ends. We could `pop` and do this implicitly, but we can also track this
down and emit a single instruction to decrement the stack pointer, which will
result in better code.
* Introduce a differentiator between addresses relative or absolute to the stack
pointer. When passing arguments in the stack -- assuming that SP at that point
is aligned for the function call -- we should store the arguments relative to
the absolute position of the stack pointer and when addressing a memory entry in
the Wasm value stack, we should use an address relative to the offset and the
position of the stack pointer.
* Simplify tracking of the stack space needed for emitting a function call
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