Improved memory usage in Espruino

I'm gonna try and fix this tonight.

I think it can be fixed with judicious use of parenthesis

Edit: Okay, yeah, I see the problem. It looks like that was written for an unsigned 32-bit integer (the code was ported from arduino), and still has workarounds for handling the large numbers.

var p = Math.round(B7 < 0x80000000 ? (B7 << 1) / B4 : (B7 / B4) << 1);

In my tests at ambient pressure, B7 is 1.6 billion or so - so when it left-shifts it, it turns into a negative number.

In at least the current ambient conditions, changing that to

var p=Math.round((B7/B4) << 1);

seems to work, but I don't think it will work at all pressures - I fear that B7 can get large enough that it could roll over to a negative number on the line before that one (since they test whether it's greater than 0x80000000, which is the point at which it would have already rolled over)