Support floating point inputs and outputs in the benchmarks

[axic]

fizzy-bench aka test/bench/bench.cpp currently requires all the inputs and outputs to be integers. Extend this to support floating point.

This also needs changes in test/utils/wasm_engine.hpp and its implementations. This part was attempted in #507, which can be used as a reference.

Lastly change the ramanujan_pi and taylor_pi benchmarks to output floating point.