Change syntax for blocks, and allow them to carry a lifetime

[madsmtm]

Part of #168.

Change the syntax to Block<dyn Fn(Args) -> R>, which more clearly describes the required signature, and allows us to introduce lifetimes to correctly mark noescape/NS_NOESCAPE blocks, and make functions taking those safe.

// Before
fn takes_block(block: &Block<(u32,), u8>) { ... }
unsafe fn takes_noescape_block(block: &Block<(u32, u32), ()>) { ... }

// After
fn takes_block(block: &Block<dyn Fn(u32) -> u8>) { ... }
fn takes_noescape_block(block: &Block<dyn Fn(u32, u32) + '_>) { ... }