Update vendored dependencies

Nothing important, but just working out this workflow.
diff --git a/third-party/BUCK b/third-party/BUCK
index 1c1bf3d..5c0e90d 100644
--- a/third-party/BUCK
+++ b/third-party/BUCK
@@ -170,7 +170,7 @@
 
 rust_library(
     name = "syn",
-    srcs = glob(["vendor/syn-1.0.14/src/**"]),
+    srcs = glob(["vendor/syn-1.0.15/src/**"]),
     visibility = ["PUBLIC"],
     features = [
         "clone-impls",
diff --git a/third-party/BUILD b/third-party/BUILD
index 11b4257..ceae8e2 100644
--- a/third-party/BUILD
+++ b/third-party/BUILD
@@ -176,7 +176,7 @@
 
 rust_library(
     name = "syn",
-    srcs = glob(["vendor/syn-1.0.14/src/**"]),
+    srcs = glob(["vendor/syn-1.0.15/src/**"]),
     crate_features = [
         "clone-impls",
         "derive",
diff --git a/third-party/Cargo.lock b/third-party/Cargo.lock
index fda2025..3844959 100644
--- a/third-party/Cargo.lock
+++ b/third-party/Cargo.lock
@@ -160,9 +160,9 @@
 
 [[package]]
 name = "libc"
-version = "0.2.66"
+version = "0.2.67"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d515b1f41455adea1313a4a2ac8a8a477634fbae63cc6100e3aebb207ce61558"
+checksum = "eb147597cdf94ed43ab7a9038716637d2d1bf2bc571da995d0028dec06bd3018"
 
 [[package]]
 name = "link-cplusplus"
@@ -297,9 +297,9 @@
 
 [[package]]
 name = "syn"
-version = "1.0.14"
+version = "1.0.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "af6f3550d8dff9ef7dc34d384ac6f107e5d31c8f57d9f28e0081503f547ac8f5"
+checksum = "7a0294dc449adc58bb6592fff1a23d3e5e6e235afc6a0ffca2657d19e7bbffe5"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -366,9 +366,9 @@
 
 [[package]]
 name = "trybuild"
-version = "1.0.21"
+version = "1.0.23"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3f5b3f750c701725331ac78e389b5d143b7d25f6b6ffffd0d419759a9063ac5f"
+checksum = "26ff1b18659a2218332848d76ad1c867ce4c6ee37b085e6bc8de9a6d11401220"
 dependencies = [
  "glob",
  "lazy_static",