Add borsh dep to Cargo manifest

Author: sug0

Cargo.toml

@@ -19,6 +19,7 @@ equivalent = { version = "1.0", default-features = false }
 arbitrary = { version = "1.0", optional = true, default-features = false }
 quickcheck = { version = "1.0", optional = true, default-features = false }
 serde = { version = "1.0", optional = true, default-features = false }
+borsh = { version = "1.2", optional = true, default-features = false }
 rayon = { version = "1.5.3", optional = true }
 
 # Internal feature, only used when building as part of rustc,
@@ -41,7 +42,7 @@ serde_derive = "1.0"
 
 [features]
 default = ["std"]
-std = []
+std = ["borsh?/std"]
 
 # for testing only, of course
 test_debug = []
@@ -54,7 +55,7 @@ no-dev-version = true
 tag-name = "{{version}}"
 
 [package.metadata.docs.rs]
-features = ["arbitrary", "quickcheck", "serde", "rayon"]
+features = ["arbitrary", "quickcheck", "serde", "borsh", "rayon"]
 rustdoc-args = ["--cfg", "docsrs"]
 
 [workspace]