When using C libraries, C99 designator list initialization is often
times used to initialize data structure.
While `std.mem.zeroes` and manually assigning to each field can
achieve the same result, it is much more verbose then the equivalent
C code:
```zig
usingnamespace @cImport({
@cInclude("sokol_app.h");
});
// Using `std.mem.zeroes` and manual assignment.
var app_desc = std.mem.zeroes(sapp_desc);
app_desc.init_cb = init;
app_desc.frame_cb = frame;
app_desc.cleanup_cb = cleanup;
app_desc.width = 400;
app_desc.height = 300;
app_desc.window_name = "no default init";
// Using `std.mem.defaultInit`.
var app_desc = std.mem.defaultInit(sapp_desc, .{
.init_cb = init,
.frame_cb = frame,
.cleanup_cb = cleanup,
.width = 400,
.height = 300,
.window_name = "default init"
});
```
The `std.mem.defaultInit` aims to solve this problem by zero
initializing all fields of the given struct to their zero, or default
value if any. Each field mentionned in the `init` variable is then
assigned to the corresponding field in the struct.
If a field is a struct, and an initializer for it is present, it is
recursively initialized.
Given that the previous design would require the use of a default
allocator to have `ArgIterator.init()` work in WASI, and since in
Zig we're trying to avoid default allocators, I've changed the design
slightly in that now `init()` is a compile error in WASI, and instead
in its message it points to `initWithAllocator(*mem.Allocator)`.
The latter by virtue of requiring an allocator as an argument can
safely be used in WASI as well as on other OSes (where the allocator
argument is simply unused). When using `initWithAllocator` it is then
natural to remember to call `deinit()` after being done with the
iterator. Also, to make use of this, I've also added `argsWithAllocator`
function which is equivalent to `args` minus the requirement of supplying
an allocator and being fallible.
Finally, I've also modified the WASI only test `process.ArgWasiIterator`
to test all OSes.
This commit pulls WASI specific implementation of args extraction
from the runtime from `process.argsAlloc` and `process.argsFree`
into a new iterator struct `process.ArgIteratorWasi`. It also
integrates the struct with platform-independent `process.ArgIterator`.
I'm not sure why I disabled them when landing extended Wasm/WASI
support, but they pass the parser tests just fine now, so I'm gonna
go ahead and re-enable them.
