[wasm] Double default initial heap size

[kg]

While we attempt to calculate a better initial heap size based on how big our DLLs are, the default minimum threshold is too low. In my testing with a very small app, the heap grows to around double this size just during the process of initializing sgen, and it does so in multiple steps instead of one big step. We may want to go even further than this, or look into a way of calculating the right value that takes things like sgen into account.

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