Simplify type definition

We can remove the dummy generic M as suggested by tycho01 [here](microsoft/TypeScript#6229 (comment)).

Author: mstn

src/index.d.ts

@@ -1,7 +1,7 @@
 // we use numeric literal types for length as TS 2.7 does for fixed size tuples
 // N is the length of the array
 // T is the type of array elements
-// M is a dummy type inizialized to '0', we need it to trick the compiler
-export type FixedSizeArray<N extends number, T, M extends string = '0'> = {
-    [k in M]: any;
-} & { length: N } & ReadonlyArray<T>;
+export type FixedSizeArray<N extends number, T> = {
+    0: any;
+    length: N;
+} & ReadonlyArray<T>;