Two extra annotations: donotinclude and typedefto

[detly]

This adds two extra annotations:

donotinclude

Currently the code generator assumes that every schema import statement requires a #include directive in the corresponding generated code. However this is not always the case eg. a schema that contains only annotation declarations doesn't generate any code. The best example of this is c.capnp.h itself (see #22) but also other language bindings eg. Rust's rust.capnp contains only annotations too.

Ideally the generator could detect this and suppress #include directives for any such schema file, however I couldn't figure out how to do this without multiple passes over input files and massive refactoring. The next best solution I could come up with was to have an annotation at the file level that declares that a particular ID should not result in a #include statement. For example, in the user's schema, they would have:

using C = import "/capnp/c.capnp";

$C.donotinclude(0xc0183dd65ffef0f3);

Note that 0xc0183dd65ffef0f3 is the file ID for c.capnp. It does not matter whether the donotinclude annotation comes before or after the import statement. Multiple annotations are fine:

using C = import "/capnp/c.capnp";
using Rust = import "rust.capnp";

$C.donotinclude(0xc0183dd65ffef0f3);
$C.donotinclude(0x83b3c14c3c8dd083);

typedefto

This is purely cosmetic. This annotation lets you declare that a struct or enum must have a typedef generated for it with a given name. For example:

enum FanSpeed $C.typedefto("fan_speed_t") {
    high @0;
    low @1;
}

...will result in the following typedef:

typedef enum FanSpeed fan_speed_t;

I did this so that code can look a little more consistent with local style guides at the calling sites. Eventually it might be nice to implement a full camelCase/snake_case conversion, but that is a great deal more work.

It does not support doing this for the "which" enums created for unions.