Add benchmark GHA

.github/workflows/benchmark.yml

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+name: benchmarks
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+on:
+  pull_request:
+    branches:
+      - master
+      - main
+    paths-ignore:
+      - 'LICENSE.md'
+      - 'README.md'
+      - 'docs/**'
+      - 'format/**'
+      - 'test/**'
+jobs:
+    benchmarks:
+        runs-on: ubuntu-latest
+        strategy:
+          fail-fast: false
+        steps:
+          - uses: actions/checkout@v3
+          - uses: julia-actions/setup-julia@v1
+            with:
+              version: "1"
+          - uses: julia-actions/cache@v1
+          - uses: julia-actions/julia-buildpkg@v1
+          - name: Benchmark
+            run: |
+              git fetch origin +:refs/remotes/origin/HEAD
+              julia --project=benchmark/ -e '
+                  using Pkg
+                  Pkg.develop(PackageSpec(path=pwd()))
+                  Pkg.instantiate()'
+              # Pkg.update() allows us to benchmark even when dependencies/compat requirements change
+              julia --project=benchmark/ -e '
+                  using PkgBenchmark, Loess
+                  juliacmd = `$(Base.julia_cmd()) -O3 -e "using Pkg; Pkg.update()"`
+                  config = BenchmarkConfig(; id="origin/HEAD", juliacmd)
+                  export_markdown(stdout, judge(Loess, config; verbose=true))'

benchmark/Project.toml

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+[deps]
+BenchmarkTools = "6e4b80f9-dd63-53aa-95a3-0cdb28fa8baf"
+Loess = "4345ca2d-374a-55d4-8d30-97f9976e7612"
+PkgBenchmark = "32113eaa-f34f-5b0d-bd6c-c81e245fc73d"
+Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
+
+[compat]
+BenchmarkTools = "1"
+PkgBenchmark = "0.2"
+julia = "1"

benchmark/benchmarks.jl

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+using BenchmarkTools, Loess, Random
+
+const SUITE = BenchmarkGroup()
+
+SUITE["random"] = BenchmarkGroup()
+
+for i in 2:6
+    n = 10^i
+    x = rand(MersenneTwister(42), n)
+    y = sqrt.(x)
+    SUITE["random"][string(n)] = @benchmarkable loess($x, $y)
+end
+
+SUITE["ties"] = BenchmarkGroup()
+let
+    x = repeat([π/4*i for i in -20:20], inner=101)
+    y = sin.(x)
+    SUITE["ties"]["sine"] = @benchmarkable loess($x, $y; span=0.2)
+end