M:N threading

* add std.atomic.QueueMpsc.isEmpty
 * make std.debug.global_allocator thread-safe
 * std.event.Loop: now you have to choose between
   - initSingleThreaded
   - initMultiThreaded
 * std.event.Loop multiplexes coroutines onto kernel threads
 * Remove std.event.Loop.stop. Instead the event loop run() function
   returns once there are no pending coroutines.
 * fix crash in ir.cpp for calling methods under some conditions
 * small progress self-hosted compiler, analyzing top level declarations
 * Introduce std.event.Lock for synchronizing coroutines
 * introduce std.event.Locked(T) for data that only 1 coroutine should
   modify at once.
 * make the self hosted compiler use multi threaded event loop
 * make std.heap.DirectAllocator thread-safe

See #174

TODO:
 * call sched_getaffinity instead of hard coding thread pool size 4
 * support for Windows and MacOS
 * #1194
 * #1197

std/debug/index.zig

@@ -11,6 +11,11 @@ const builtin = @import("builtin");
 
 pub const FailingAllocator = @import("failing_allocator.zig").FailingAllocator;
 
+pub const runtime_safety = switch (builtin.mode) {
+    builtin.Mode.Debug, builtin.Mode.ReleaseSafe => true,
+    builtin.Mode.ReleaseFast, builtin.Mode.ReleaseSmall => false,
+};
+
 /// Tries to write to stderr, unbuffered, and ignores any error returned.
 /// Does not append a newline.
 /// TODO atomic/multithread support
@@ -1098,7 +1103,7 @@ fn readILeb128(in_stream: var) !i64 {
 
 /// This should only be used in temporary test programs.
 pub const global_allocator = &global_fixed_allocator.allocator;
-var global_fixed_allocator = std.heap.FixedBufferAllocator.init(global_allocator_mem[0..]);
+var global_fixed_allocator = std.heap.ThreadSafeFixedBufferAllocator.init(global_allocator_mem[0..]);
 var global_allocator_mem: [100 * 1024]u8 = undefined;
 
 // TODO make thread safe