[NFC] Pass position as an argument in WasmBinaryReader

[tlively]

Remove the pos member variable that keeps track of the current
position in the input buffer and instead thread the position through all
the calls to the parsing methods. In a future PR that paralellizes the
parsing of function bodies, this will enable the parsing methods to be
called in parallel without unsafely modifying shared state. Using a
parameter for every method rather than just those used while parsing
function bodies makes it impossible by construction to have data races
on the position.