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/json_tests/test_common.rs

@@ -52,7 +52,8 @@ pub fn run_test_file(path: &Path, runner: fn (json_data: &[u8]) -> Vec<String>)
 	let mut file = File::open(&path).expect("Error opening test file");
 	file.read_to_end(&mut data).expect("Error reading test file");
 	let results = runner(&data);
-	assert!(results.is_empty());
+	let empty: [String; 0] = [];
+	assert_eq!(results, empty);
 }
 
 macro_rules! test {