Decentralize contract entry points

[webmaster128]

Right now there are 3 places where entry points (i.e. Wasm exported functions) are created:

1. Any use cosmwasm_std creates cosmwasm_vm_version_4, allocate, deallocate
2. The create_entry_points! macro creates init, handle, query
3. The create_entry_points_with_migration! macro creates init, handle, query, migrate

This model does not scale. But it is probably not great anyways to require this central place where all entry points are collected.

Instead we can decorate the entrypoints directly with something like this:

#[entry_point_init]
pub fn init(
    deps: DepsMut,
    _env: Env,
    info: MessageInfo,
    msg: InitMsg,
) -> Result<InitResponse, HackError> {

This becomes very useful with #692 and #693.