Generic set_value API

From: !198 (closed):

@gpeskens: What I've been mentally toying with though is a generic rist_set_value API, perhaps something like this:

int rist_set_value(struct rist_ctx *ctx, enum value_type type, void *val1, void *val2)
{

}

That would be extendable in the future, keeping the API surface smaller.