Add argument for fillFn to Random.init

As suggested by @leecannon, this provides more flexibility to the
`Random` interface. For exmaple, this allows for an implementation to
provide multiple different fill functions.

lib/std/rand.zig

@@ -32,17 +32,16 @@ pub const Random = struct {
     ptr: *c_void,
     fillFn: fn (ptr: *c_void, buf: []u8) void,
 
-    pub fn init(pointer: anytype) Random {
+    pub fn init(pointer: anytype, comptime fillFn: fn (ptr: @TypeOf(pointer), buf: []u8) void) Random {
         const Ptr = @TypeOf(pointer);
         assert(@typeInfo(Ptr) == .Pointer); // Must be a pointer
         assert(@typeInfo(Ptr).Pointer.size == .One); // Must be a single-item pointer
         assert(@typeInfo(@typeInfo(Ptr).Pointer.child) == .Struct); // Must point to a struct
-        assert(std.meta.trait.hasFn("fill")(@typeInfo(Ptr).Pointer.child)); // Struct must provide the `fill` function
         const gen = struct {
             fn fill(ptr: *c_void, buf: []u8) void {
                 const alignment = @typeInfo(Ptr).Pointer.alignment;
                 const self = @ptrCast(Ptr, @alignCast(alignment, ptr));
-                self.fill(buf);
+                fillFn(self, buf);
             }
         };
 
@@ -333,7 +332,7 @@ const SequentialPrng = struct {
     }
 
     pub fn random(self: *Self) Random {
-        return Random.init(self);
+        return Random.init(self, fill);
     }
 
     pub fn fill(self: *Self, buf: []u8) void {

lib/std/rand/Gimli.zig

@@ -21,7 +21,7 @@ pub fn init(secret_seed: [secret_seed_length]u8) Gimli {
 }
 
 pub fn random(self: *Gimli) Random {
-    return Random.init(self);
+    return Random.init(self, fill);
 }
 
 pub fn fill(self: *Gimli, buf: []u8) void {

lib/std/rand/Isaac64.zig

@@ -31,7 +31,7 @@ pub fn init(init_s: u64) Isaac64 {
 }
 
 pub fn random(self: *Isaac64) Random {
-    return Random.init(self);
+    return Random.init(self, fill);
 }
 
 fn step(self: *Isaac64, mix: u64, base: usize, comptime m1: usize, comptime m2: usize) void {

lib/std/rand/Pcg.zig

@@ -22,7 +22,7 @@ pub fn init(init_s: u64) Pcg {
 }
 
 pub fn random(self: *Pcg) Random {
-    return Random.init(self);
+    return Random.init(self, fill);
 }
 
 fn next(self: *Pcg) u32 {

lib/std/rand/Sfc64.zig

@@ -24,7 +24,7 @@ pub fn init(init_s: u64) Sfc64 {
 }
 
 pub fn random(self: *Sfc64) Random {
-    return Random.init(self);
+    return Random.init(self, fill);
 }
 
 fn next(self: *Sfc64) u64 {

lib/std/rand/Xoroshiro128.zig

@@ -17,7 +17,7 @@ pub fn init(init_s: u64) Xoroshiro128 {
 }
 
 pub fn random(self: *Xoroshiro128) Random {
-    return Random.init(self);
+    return Random.init(self, fill);
 }
 
 fn next(self: *Xoroshiro128) u64 {

lib/std/rand/Xoshiro256.zig

@@ -19,7 +19,7 @@ pub fn init(init_s: u64) Xoshiro256 {
 }
 
 pub fn random(self: *Xoshiro256) Random {
-    return Random.init(self);
+    return Random.init(self, fill);
 }
 
 fn next(self: *Xoshiro256) u64 {