std.posix.iovec: use .base and .len instead of .iov_base and .iov_len

[truemedian]

Also uses the opportunity to apply this change to windows' WSABUF.

This removes the posix-y field names in favor of more concise names.

[andrewrk]

The naming convention is to keep the posix name, but remove the prefix. I understand that this type fulfills the same purpose as a Zig language slice, however, I don't see why that means an exception to the naming convention.

[truemedian]

I decided to use the zig naming here made more sense because the primary goal was to be able to interop between posix's iovec and windows's WSABUF. posix calls it base, windows calls it buf, so I decided to adopt the zig name here.

Would you prefer to use base for posix.iovec and WSABUF instead of ptr?

[andrewrk]

WSABUF should continue to use len and buf, matching the API definitions in windows.h, and iovec should continue to use base and len, matching the POSIX API definitions.

The cross-platform abstractions offered by the zig standard library must be implemented on top of that.