For example, a situation like this is allowed
```zig
extern "c" var stderrp: c_int;
```
In this case, `Module.Var` wrapping `stderrp` will have `lib_name`
populated with the library name where this import is expected.
`ExternFn` will contain a maybe-lib-name if it was defined with
the `extern` keyword like so
```zig
extern "c" fn write(usize, usize, usize) usize;
```
`lib_name` will live as long as `ExternFn` decl does.
fieldVal handles pointer to pointer to array. This can happen for
example, if a pointer to an array is used as the condition expression of
a for loop.
resolveStructFully handles tuples (by doing nothing).
fixed Type comparison for tuples to handle comptime fields properly.
* resolve_inferred_alloc now gives a proper mutability attribute to the
corresponding alloc instruction. Previously, it would fail to mark
things const.
* slicing: fix the detection for when the end index equals the length
of the underlying object. Previously it was using `end - start` but
it should just use the end index directly. It also takes into account
when slicing a comptime-known slice.
* `Type.sentinel`: fix not handling all slice tags
Singular tests (such as in the bug ones) are moved to top level with exclusions for non-passing backends.
The big behavior tests such as array_llvm and slice are moved to the inner scope with the C backend disabled.
They all pass for the wasm backend now
We now calculate the total stack size required for the current frame.
The default alignment of the stack is 16 bytes, and will be overwritten when the alignment
of a given type is larger than that.
After we have generated all instructions for the body, we calculate the total stack size
by forward aligning the stack size while accounting for the max alignment.
We then insert a prologue into the body, where we substract this size from the stack pointer
and save it inside a bottom stackframe local. We use this local then, to calculate
the stack pointer locals of all variables we allocate into the stack.
In a future iteration we can improve this further by storing the offsets as a new `stack_offset` `WValue`.
This has the benefit of not having to spend runtime cost of storing those offsets, but instead we append
those offsets whenever we need the value that lives in the stack.
Implements the instruction `vector_init` for structs and arrays.
For arrays, it checks if the element must be passed by reference or not.
When not, it can simply use the `offset` field of a store instruction to copy the values
into the array. When it is byref, it will move the pointer by the element size, and then perform
a store operation. This ensures types like structs will be moved into the right position.
For structs we will always move the pointer, as we currently cannot verify if all fields are
not by ref.
This implements lowering elem_ptr for decl's and constants.
To generate the correct pointer, we perform a relocation by using the addend
that represents the offset. The offset is calculated by taking the element's size
and multiplying that by the index.
For constants this generates a single immediate instruction, and for decl's
this generates a single pointer address.
* pad out (non-packed) struct fields when lowering to bytes to be
saved in the binary - prior to this change, fields would be
saved at non-aligned addresses leading to wrong accesses
* add a matching test case to `behavior/struct.zig` tests
* fix offset to field calculation in `struct_field_ptr` on `x86_64`
