combine std.build and std.build.Builder into std.Build

I've been wanting to do this for along time.

lib/std/Build/CheckFileStep.zig

@@ -0,0 +1,51 @@
+const std = @import("../std.zig");
+const Step = std.Build.Step;
+const fs = std.fs;
+const mem = std.mem;
+
+const CheckFileStep = @This();
+
+pub const base_id = .check_file;
+
+step: Step,
+builder: *std.Build,
+expected_matches: []const []const u8,
+source: std.Build.FileSource,
+max_bytes: usize = 20 * 1024 * 1024,
+
+pub fn create(
+    builder: *std.Build,
+    source: std.Build.FileSource,
+    expected_matches: []const []const u8,
+) *CheckFileStep {
+    const self = builder.allocator.create(CheckFileStep) catch unreachable;
+    self.* = CheckFileStep{
+        .builder = builder,
+        .step = Step.init(.check_file, "CheckFile", builder.allocator, make),
+        .source = source.dupe(builder),
+        .expected_matches = builder.dupeStrings(expected_matches),
+    };
+    self.source.addStepDependencies(&self.step);
+    return self;
+}
+
+fn make(step: *Step) !void {
+    const self = @fieldParentPtr(CheckFileStep, "step", step);
+
+    const src_path = self.source.getPath(self.builder);
+    const contents = try fs.cwd().readFileAlloc(self.builder.allocator, src_path, self.max_bytes);
+
+    for (self.expected_matches) |expected_match| {
+        if (mem.indexOf(u8, contents, expected_match) == null) {
+            std.debug.print(
+                \\
+                \\========= Expected to find: ===================
+                \\{s}
+                \\========= But file does not contain it: =======
+                \\{s}
+                \\
+            , .{ expected_match, contents });
+            return error.TestFailed;
+        }
+    }
+}