This implements stage1 parser support for anonymous struct literal
syntax (see #685), as well as semantic analysis support for anonymous
struct literals and anonymous list literals (see #208). The semantic
analysis works when there is a type coercion in the result location;
inferring the struct type based on the values in the literal is not
implemented yet. Also remaining to do is zig fmt support for this new
syntax and documentation updates.
* All the data types from `@import("builtin")` are moved to
`@import("std").builtin`. The target-related types are moved
to `std.Target`. This allows the data types to have methods, such as
`std.Target.current.isDarwin()`.
* `std.os.windows.subsystem` is moved to
`std.Target.current.subsystem`.
* Remove the concept of the panic package from the compiler
implementation. Instead, `std.builtin.panic` is always the panic
function. It checks for `@hasDecl(@import("root"), "panic")`,
or else provides a default implementation.
This is an important step for multibuilds (#3028). Without this change,
the types inside the builtin namespace look like different types, when
trying to merge builds with different target settings. With this change,
Zig can figure out that, e.g., `std.builtin.Os` (the enum type) from one
compilation and `std.builtin.Os` from another compilation are the same
type, even if the target OS value differs.
