Integrate a new test runner

This change brings in work done in a separate repository to create a new benchmark runner for sightglass. The original code is retained with a `legacy` moniker where appropriate; the new runner uses `next`. The `README.md` now contains a description of the different components present in the repository and links to documentation or implementation for each. In writing this, it clarified for me that a reorganization (in a future PR) is needed.

The new runner employs a different paradigm than the old one:
 - benchmarks are Wasm files, not shared libraries, built using Dockerfiles and committed in-tree (accompanied by their WAT pair) under `benchmarks-next`; they   import `bench.start` and `bench.end` functions which are placed around the section of code to measure
 - engines (i.e. Wasm VMs) are shared libraries, like the one introduced in bytecodealliance/wasmtime#2437, allowing the runner to control the compilation, instantiation, and execution of the Wasm benchmark
 - the building and running of the benchmarks is controlled with a CLI tool, see `cargo +nightly run -- --help` and `docs/next.md` for more information

.github/workflows/sg-history.yml

@@ -21,6 +21,6 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Build
-      run: cargo build --verbose
+      run: cargo build -p sg-history --verbose
     - name: Run tests
-      run: cargo test --verbose
+      run: cargo test -p sg-history --verbose

.github/workflows/sightglass.yml

@@ -8,6 +8,7 @@ on:
 
 env:
   CARGO_TERM_COLOR: always
+  RUST_LOG: info
 
 jobs:
   build:
@@ -16,13 +17,11 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: Build
+    - name: Install nightly
       uses: actions-rs/toolchain@v1
       with:
         toolchain: nightly
-    - run: cargo build --verbose
-    - name: Run tests
-      uses: actions-rs/toolchain@v1
-      with:
-        toolchain: nightly
-    - run: cargo test --verbose
+    - name: Build all
+      run: cargo +nightly build --verbose --all
+    - name: Test all
+      run: cargo +nightly test --verbose --all