[roll third_party/pigweed] pw_varint: Update Rust API to return number of bytes written

Returning number of bytes written, like the `Read` and `Write` traits,
allows for more flexible use of varint functions.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/156450

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 05d4532e72b5d42..1ea4e1ecdbba8a7
Roller-URL: https://ci.chromium.org/b/8775272785189008145
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ie9283d40ece5c2dcabfde716e4630519e39e3dd4
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/156610
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 05d4532..1ea4e1e 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 05d4532e72b5d429792016c670deef3581a5ff43
+Subproject commit 1ea4e1ecdbba8a73c4d3bd2b5e4107cc9764abb9