fix(gas snapshot format): detail symbol meaning (#28)

provide more context for symbolic output from gas runner in forge

Author: sambacha

README.md

@@ -216,6 +216,8 @@ FooTest:testSetFoo() (gas: 64071)
 BarTest:testFuzzCurrentBar(uint256) (runs: 256, μ: 408, ~: 423)
 ```
 
+runs: {runs}, μ: {mean_gas}, ~: {median_gas})
+
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 ### JSON-Schema definition