Support a fieldof type.

[Fruneau]

In some cases, we may want to have a function that read one field of an object identified by a parameter. Here is a small synthetic use case, for the purpose of the discussion:

interface Help {
    brief: string;
    detailed: string;
    contextual: string;
}

function getHelpString(help: Help, format: string): string
{
    return help[format];
}

In that particular case, the type checker cannot ensure that we are doing things right. In order to make the type checker happy, we need a way to tell it that format is actually part of an implicit enumeration that only contains the names of the fields of the interface Help. This could be done by introducing a new type inference operator, for example fieldof.

function getHelpString(help: Help, format: fieldof Help): string
{
    return help[format];
}

This may not look like a very common use case, but I hoped to have such an operator in order to port incrementally a large amount of javascript code to typescript.

[jbrantly]

Duplicate of #394

[DanielRosenwasser]

Thanks for the suggestrion @Fruneau, and thanks for catching that @jbrantly. Closing in favor of #394.