Use envvars in benchmark for controlling PGF/LPGF. Add readme.

testsuite/lpgf/README.md

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+# LPGF testsuite & benchmark
+
+## Test
+
+LPGF must be equivalent to PGF in terms of linearisation output.
+
+Possible exceptions:
+- No handling of variants (design choice)
+- Rendering of missing fucntions
+
+## Benchmark
+
+### Compilation
+
+Comparing PGF, LPGF along following criteria:
+
+- Time
+- Memory
+- Binary file size
+
+### Runtime (linearisation)
+
+Comparing PGF, PGF2, LPGF along following criteria:
+
+- Time
+- Memory
+
+### Running
+
+```
+stack build --test --bench --no-run-tests --no-run-benchmarks && time stack bench
+stack build --test --bench --no-run-tests --no-run-benchmarks && time PGF_ONLY=1 stack bench
+stack build --test --bench --no-run-tests --no-run-benchmarks && time LPGF_ONLY=1 stack bench
+```