runtime: add exit() builtin function

SPEC.md

@@ -168,6 +168,7 @@ These are the right primitives, but we can build much more sophisticated systems
 - `urand(length) => string`: a string of given length containing random bits, safe for cryptography work
 - `time() => number`: number of seconds in floating point in UNIX epoch.
 - `exec(string, [list], string, callback) => callback`: Exec the command at a given path with given arguments, with a given stdin, call given callback with stdout when exited.
+- `exit(number)`: Exit the current process with the given exit code.
 
 ### Math
 

pkg/ink/runtime.go

@@ -66,6 +66,7 @@ func (ctx *Context) LoadEnvironment() {
 	ctx.LoadFunc("time", inkTime)
 	ctx.LoadFunc("wait", inkWait)
 	ctx.LoadFunc("exec", inkExec)
+	ctx.LoadFunc("exit", inkExit)
 
 	// math
 	ctx.LoadFunc("sin", inkSin)
@@ -1362,6 +1363,29 @@ func inkExec(ctx *Context, in []Value) (Value, error) {
 	}, nil
 }
 
+func inkExit(ctx *Context, in []Value) (Value, error) {
+	if len(in) < 1 {
+		return nil, Err{
+			ErrRuntime,
+			"exit() takes one argument",
+		}
+	}
+
+	code, isNum := in[0].(NumberValue)
+
+	if !isNum {
+		return nil, Err{
+			ErrRuntime,
+			"argument to exit() must be an exit code number",
+		}
+	}
+
+	os.Exit(int(float64(code)))
+
+	// unreachable
+	return Null, nil
+}
+
 func inkSin(ctx *Context, in []Value) (Value, error) {
 	if len(in) < 1 {
 		return nil, Err{