build(deps): bump sigma_fun from `9e5e193` to `b511ab5`

Bumps [sigma_fun](https://github.com/LLFourn/secp256kfun) from `9e5e193` to `b511ab5`.
- [Release notes](https://github.com/LLFourn/secp256kfun/releases)
- [Commits](9e5e193347...b511ab5f83)

---
updated-dependencies:
- dependency-name: sigma_fun
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Cargo.lock

@@ -980,7 +980,7 @@ checksum = "5caaa75cbd2b960ff1e5392d2cfb1f44717fffe12fc1f32b7b5d1267f99732a6"
 [[package]]
 name = "ecdsa_fun"
 version = "0.7.1"
-source = "git+https://github.com/LLFourn/secp256kfun#9e5e193347bcffc086d35fa5d8a5a1d5081adf64"
+source = "git+https://github.com/LLFourn/secp256kfun#b511ab5f83aa0ae21f10460ada56f535ad3a6d57"
 dependencies = [
  "bincode",
  "rand_chacha 0.3.1",
@@ -3428,7 +3428,7 @@ dependencies = [
 [[package]]
 name = "secp256kfun"
 version = "0.7.1"
-source = "git+https://github.com/LLFourn/secp256kfun#9e5e193347bcffc086d35fa5d8a5a1d5081adf64"
+source = "git+https://github.com/LLFourn/secp256kfun#b511ab5f83aa0ae21f10460ada56f535ad3a6d57"
 dependencies = [
  "digest 0.10.5",
  "rand_core 0.6.2",
@@ -3625,7 +3625,7 @@ dependencies = [
 [[package]]
 name = "sigma_fun"
 version = "0.4.1"
-source = "git+https://github.com/LLFourn/secp256kfun#9e5e193347bcffc086d35fa5d8a5a1d5081adf64"
+source = "git+https://github.com/LLFourn/secp256kfun#b511ab5f83aa0ae21f10460ada56f535ad3a6d57"
 dependencies = [
  "curve25519-dalek-ng",
  "digest 0.10.5",