Break out the impl to a lib

This will make it easier to make a C library.

build.zig

@@ -15,6 +15,12 @@ pub fn build(b: *std.Build) void {
     // set a preferred release mode, allowing the user to decide how to optimize.
     const optimize = b.standardOptimizeOption(.{});
 
+    const lib_mod = b.createModule(.{
+        .root_source_file = b.path("src/root.zig"),
+        .target = target,
+        .optimize = optimize,
+    });
+
     // We will also create a module for our other entry point, 'main.zig'.
     const exe_mod = b.createModule(.{
         // `root_source_file` is the Zig "entry point" of the module. If a module
@@ -26,9 +32,19 @@ pub fn build(b: *std.Build) void {
         .optimize = optimize,
     });
 
-    exe_mod.addImport("network", b.dependency("network", .{}).module("network"));
+    lib_mod.addImport("network", b.dependency("network", .{}).module("network"));
+
+    exe_mod.addImport("zaprus", lib_mod);
     exe_mod.addImport("clap", b.dependency("clap", .{}).module("clap"));
 
+    const lib = b.addLibrary(.{
+        .linkage = .static,
+        .name = "zaprus",
+        .root_module = lib_mod,
+    });
+
+    b.installArtifact(lib);
+
     // This creates another `std.Build.Step.Compile`, but this one builds an executable
     // rather than a static library.
     const exe = b.addExecutable(.{