Add CI job to run cargo-semver-checks (#1323)

### What
Add CI job to run cargo-semver-checks that will error if we make a
breaking change to a patch or minor release.

### Why
To monitor whether the changes being made to the soroban-sdk and its
exports are a match for the version being targeted. The
soroban-env-common and some of the guest and host crates are reexported
by the soroban-sdk and essentially part of its API.

Due to issues outlined in the comment below these checks cannot all be
done on the soroban-sdk:
-
stellar/rs-soroban-sdk#1202 (comment)

Also, independent of the above not breaking semver rules is relatively
important in the Rust ecosystem because cargo favors selecting newer
(maximum) instead of older (minimum) versions. While lock files exist,
they aren't respected by default in some contexts like `cargo install`
meaning introducing breaking changes into a crate can break downstream
tools. (We avoid this on the products we publish by pinning exact
versions of the env, but that is not common practice in the Rust
ecosystem.)

Close stellar/rs-soroban-sdk#1195

---------

Co-authored-by: Graydon Hoare <graydon@pobox.com>

.github/workflows/rust.yml

@@ -13,7 +13,7 @@ jobs:
     # temporarily disabling publish-dry-run, it appears to be broken with recent
     # changes to dependency specifications?
     # needs: [fmt, cackle, cargo-deny, check-git-rev-deps, build-and-test, publish-dry-run]
-    needs: [fmt, cackle, cargo-deny, check-git-rev-deps, build-and-test]
+    needs: [fmt, cackle, cargo-deny, check-git-rev-deps, semver-checks, build-and-test]
     runs-on: ubuntu-latest
     steps:
     - if: contains(needs.*.result, 'failure') || contains(needs.*.result, 'cancelled')
@@ -63,6 +63,17 @@ jobs:
     - name: Check if rust-analyzer encounters any errors parsing project
       run: rust-analyzer analysis-stats . 2>&1 | (! grep ERROR)
 
+  semver-checks:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - run: rustup update
+    - uses: stellar/binaries@v18
+      with:
+        name: cargo-semver-checks
+        version: 0.27.0
+    - run: cargo semver-checks
+
   build-and-test:
     strategy:
       matrix:

soroban-simulation/Cargo.toml

@@ -10,6 +10,9 @@ readme = "../README.md"
 edition = "2021"
 rust-version.workspace = true
 
+# Temporary until we publish, to satisfy cargo-semver-checks
+publish = false
+
 [dependencies]
 # To be removed
 anyhow = { version = "1.0.75", features = [] }