Use memmap for dag cache (#6193)

* Rebase and fix compilation errors (tests not yet fixed)

* Use `debug_assert` over `assert`

* Fix tests

* Assert safety, clean up

* Fix up stale cache removal, move one assert to debug_assert

* Remove printlns

* Add licenses

* Fix benches

* Inline some no-ops in a hot loop that weren't being inlined

* Add spooky comment to make sure no-one removes the inlining annotations

* Minor cleanup

* Add option to switch between mmap and ram

* Flag ethash to use less memory when running light client

* Fix tests

* Remove todo comment (it's done)

* Replace assertion with error return

* Fix indentation

* Use union instead of `transmute`

* Fix benches

* Extract to constants

* Clean up and fix soundness holes

* Fix formatting

* Ignore missing-file errors

* Make incorrect cache size an error condition instead of a panic, remove dead code

* Fix compilation errors from rebase

* Fix compilation errors in tests

* Fix compilation errors in tests

ethcore/src/engines/basic_authority.rs

@@ -270,7 +270,7 @@ mod tests {
 	/// Create a new test chain spec with `BasicAuthority` consensus engine.
 	fn new_test_authority() -> Spec {
 		let bytes: &[u8] = include_bytes!("../../res/basic_authority.json");
-		Spec::load(::std::env::temp_dir(), bytes).expect("invalid chain spec")
+		Spec::load(&::std::env::temp_dir(), bytes).expect("invalid chain spec")
 	}
 
 	#[test]