parser: produce helpful error messages on command line errors

This is the first cut at providing human-readable context for command
line parsing failures. Since these failures are due to incorrect
input (normally produced by a human), closing the information loop at
the human layer makes a hell of a lot more sense than dumping an error
traceback with a (possibly cryptic) error name and calling it a day.

This approach doesn't print anything out by default and still depends
on the user to choose exactly how the handle and print the error
message. Errors are propagated from subcommands, though they end up
being copied, which shouldn't be strictly necessary. Maybe this can be
improved in the future. OutOfMemory has been added to ParseError to
simplify the code a bit.

The demo has been updated with a simplistic example of what presenting
error messages to the user may look like. I don't know that this
produces useful messages for every possible failure scenario, but it
does for the most common ones.

build.zig

@@ -1,11 +1,11 @@
 const std = @import("std");
 
 pub fn build(b: *std.Build) void {
-    const target: std.Build.ResolvedTarget = b.standardTargetOptions(.{});
+    const target: std.zig.CrossTarget = b.standardTargetOptions(.{});
     const optimize: std.builtin.Mode = b.standardOptimizeOption(.{});
 
     const noclip = b.addModule("noclip", .{
-        .root_source_file = .{ .path = "source/noclip.zig" },
+        .source_file = .{ .path = "source/noclip.zig" },
     });
 
     demo(b, noclip, target, optimize);
@@ -24,7 +24,7 @@ pub fn build(b: *std.Build) void {
 fn demo(
     b: *std.Build,
     noclip: *std.Build.Module,
-    target: std.Build.ResolvedTarget,
+    target: std.zig.CrossTarget,
     optimize: std.builtin.Mode,
 ) void {
     const demo_step = b.step("demo", "Build and install CLI demo program");
@@ -35,7 +35,7 @@ fn demo(
         .target = target,
         .optimize = optimize,
     });
-    exe.root_module.addImport("noclip", noclip);
+    exe.addModule("noclip", noclip);
     const install_demo = b.addInstallArtifact(exe, .{});
 
     demo_step.dependOn(&install_demo.step);

build.zig.zon

@@ -1,11 +0,0 @@
-.{
-    .name = "NOCLIP",
-    .version = "0.1.0-pre",
-    .dependencies = .{},
-    .paths = .{
-        "source",
-        "build.zig",
-        "build.zig.zon",
-        "license",
-    },
-}

source/command.zig

@@ -446,21 +446,21 @@ pub fn CommandBuilder(comptime UserContext: type) type {
                 }
 
                 _ = @Type(.{ .Struct = .{
-                    .layout = .auto,
+                    .layout = .Auto,
                     .fields = tag_fields,
                     .decls = &.{},
                     .is_tuple = false,
                 } });
 
                 _ = @Type(.{ .Struct = .{
-                    .layout = .auto,
+                    .layout = .Auto,
                     .fields = env_var_fields,
                     .decls = &.{},
                     .is_tuple = false,
                 } });
 
                 return @Type(.{ .Struct = .{
-                    .layout = .auto,
+                    .layout = .Auto,
                     .fields = fields,
                     .decls = &.{},
                     .is_tuple = false,
@@ -519,7 +519,7 @@ pub fn CommandBuilder(comptime UserContext: type) type {
                 }
 
                 return @Type(.{ .Struct = .{
-                    .layout = .auto,
+                    .layout = .Auto,
                     .fields = fields,
                     .decls = &.{},
                     .is_tuple = false,

source/meta.zig

@@ -265,7 +265,7 @@ pub const TupleBuilder = struct {
             }
 
             return @Type(.{ .Struct = .{
-                .layout = .auto,
+                .layout = .Auto,
                 .fields = &fields,
                 .decls = &.{},
                 .is_tuple = true,