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

evmbin/src/main.rs

@@ -230,7 +230,7 @@ impl Args {
 		Ok(match self.flag_chain {
 			Some(ref filename) =>  {
 				let file = fs::File::open(filename).map_err(|e| format!("{}", e))?;
-				spec::Spec::load(::std::env::temp_dir(), file)?
+				spec::Spec::load(&::std::env::temp_dir(), file)?
 			},
 			None => {
 				ethcore::ethereum::new_foundation(&::std::env::temp_dir())