add an ast smith

This generates zig ASTs from `testing.Smith` and is based off the
langref's PEG.

The choice to not build the Ast while generating and instead parsing it
afterwards makes the smith more versatile by not being tied to a single
implementation at a cost of efficiency.

Additionally, a new function `boolWeighted` was added to `Smith` due to
its frequent use in `AstSmith`.

lib/std/zig.zig

@@ -23,6 +23,7 @@ pub const primitives = @import("zig/primitives.zig");
 pub const isPrimitive = primitives.isPrimitive;
 pub const Ast = @import("zig/Ast.zig");
 pub const AstGen = @import("zig/AstGen.zig");
+pub const AstSmith = @import("zig/AstSmith.zig");
 pub const Zir = @import("zig/Zir.zig");
 pub const Zoir = @import("zig/Zoir.zig");
 pub const ZonGen = @import("zig/ZonGen.zig");
@@ -1165,6 +1166,7 @@ pub const ClangCliParam = struct {
 test {
     _ = Ast;
     _ = AstRlAnnotate;
+    _ = AstSmith;
     _ = BuiltinFn;
     _ = Client;
     _ = ErrorBundle;