Improve interface for specifying teardown function

[just-mitch]

Go from:

context.set_public_teardown_function(
    context.this_address(),
    FunctionSelector::from_signature("pay_fee_with_shielded_rebate(Field,(Field),Field)"),
    [amount, asset.to_field(), secret_hash]
);

to

FPC::at(context.this_address()).pay_fee_with_shielded_rebate(amount, asset, secret_hash).set_public_teardown_function(&mut context)

[iAmMichaelConnor]

If the intention is for this syntax to set the function pay_fee_with_shielded_rebate as the teardown function, perhaps call the method set_as_public_teardown_function (note the "as"). Otherwise I'd be confused as to when the function will actually be called.

My brain might prefer something like:

context.set_public_teardown_function(
    FPC::at(context.this_address()).pay_fee_with_shielded_rebate(amount, asset, secret_hash)
);