inline assembly improvements #215

This inline assembly does exit(0) on x86_64 linux:

    asm volatile ("syscall"
        : [ret] "={rax}" (-> usize)
        : [number] "{rax}" (60),
            [arg1] "{rdi}" (0)
        : "rcx", "r11");

Here are some flaws:

• 60 and 0 are number literals and need to be casted to a type to be valid. This causes an assertion failure in the compiler if you don't cast the number literals. Assembly syntax should include types for inputs.
• [number], [arg1], [ret] unused, and that is awkward.
• need multiple return values (see multiple expression return values, error type redesign, introduction of copyable property of types #83)
• do we really need this complicated restraint syntax? maybe we can operate on inputs and outputs.
• let's go digging into some real world inline assembly code to see the use cases.
• when we get errors from parsing assembly, we don't attach them to the offset from within the assembly string. connect inline assembly errors from zig back to the source #2080