Add structopt dependency in version 0.3.7

This patch series replaces argparse with structopt in the argument
handling code. As a first step, we need structopt as a dependency.

Import subrepo structopt/:structopt at efbdda4753592e27bc430fb01f7b9650b2f3174d
Import subrepo bitflags/:bitflags at 30668016aca6bd3b02c766e8347e0b4080d4c296
Import subrepo clap/:clap at 784524f7eb193e35f81082cc69454c8c21b948f7
Import subrepo heck/:heck at 093d56fbf001e1506e56dbfa38631d99b1066df1
Import subrepo proc-macro-error/:proc-macro-error at 6c4cfe79a622c5de8ae68557993542be46eacae2
Import subrepo proc-macro2/:proc-macro2 at d5d48eddca4566e5438e8a2cbed4a74e049544de
Import subrepo quote/:quote at 727436c6c137b20f0f34dde5d8fda2679b9747ad
Import subrepo rustversion/:rustversion at 0c5663313516263059ce9059ef81fc7a1cf655ca
Import subrepo syn-mid/:syn-mid at 5d3d85414a9e6674e1857ec22a87b96e04a6851a
Import subrepo syn/:syn at e87c27e87f6f4ef8919d0372bdb056d53ef0d8f3
Import subrepo textwrap/:textwrap at abcd618beae3f74841032aa5b53c1086b0a57ca2
Import subrepo unicode-segmentation/:unicode-segmentation at 637c9874c4fe0c205ff27787faf150a40295c6c3
Import subrepo unicode-width/:unicode-width at 3033826f8bf05e82724140a981d5941e48fce393
Import subrepo unicode-xid/:unicode-xid at 4baae9fffb156ba229665b972a9cd5991787ceb7

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diff --git a/syn/examples/heapsize/README.md b/syn/examples/heapsize/README.md
new file mode 100644
index 0000000..e789559
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+++ b/syn/examples/heapsize/README.md
@@ -0,0 +1,72 @@
+A derive macro that generates trait impls.
+
+- [`heapsize/src/lib.rs`](heapsize/src/lib.rs)
+- [`heapsize_derive/src/lib.rs`](heapsize_derive/src/lib.rs)
+- [`example/src/main.rs`](example/src/main.rs)
+
+We are deriving the `HeapSize` trait which computes an estimate of the amount of
+heap memory owned by a value.
+
+```rust
+pub trait HeapSize {
+    /// Total number of bytes of heap memory owned by `self`.
+    fn heap_size_of_children(&self) -> usize;
+}
+```
+
+The derive macro allows users to write `#[derive(HeapSize)]` on data structures
+in their program.
+
+```rust
+#[derive(HeapSize)]
+struct Demo<'a, T: ?Sized> {
+    a: Box<T>,
+    b: u8,
+    c: &'a str,
+    d: String,
+}
+```
+
+The trait impl generated by the derive macro here would look like:
+
+```rust
+impl<'a, T: ?Sized + heapsize::HeapSize> heapsize::HeapSize for Demo<'a, T> {
+    fn heap_size_of_children(&self) -> usize {
+        0 + heapsize::HeapSize::heap_size_of_children(&self.a)
+            + heapsize::HeapSize::heap_size_of_children(&self.b)
+            + heapsize::HeapSize::heap_size_of_children(&self.c)
+            + heapsize::HeapSize::heap_size_of_children(&self.d)
+    }
+}
+```
+
+The implementation of `heapsize_derive` demonstrates some attention to "spans"
+of error messages. For each subexpression in the generated code we apply the
+span of the input fragment under which we would want to trigger a compiler error
+if the subexpression fails to compile. In this example, each recursive call to
+`heap_size_of_children` is associated with the span of the corresponding struct
+field. Thus we get errors in the right place if any of the field types do not
+implement the `HeapSize` trait.
+
+```
+error[E0277]: the trait bound `std::thread::Thread: HeapSize` is not satisfied
+ --> src/main.rs:7:5
+  |
+7 |     bad: std::thread::Thread,
+  |     ^^^ the trait `HeapSize` is not implemented for `std::thread::Thread`
+```
+
+Some unstable APIs in the `proc-macro2` crate let us improve this further by
+joining together the span of the field name and the field type. There is no
+difference in our code &mdash; everything is as shown in this directory &mdash;
+but building the example crate with `cargo build` shows errors like the one
+above and building with `RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo build`
+is able to show errors like the following.
+
+```
+error[E0277]: the trait bound `std::thread::Thread: HeapSize` is not satisfied
+ --> src/main.rs:7:5
+  |
+7 |     bad: std::thread::Thread,
+  |     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `HeapSize` is not implemented for `std::thread::Thread`
+```