Implements the `start` section which will execute a given function
at startup of the program. After function execution, the _start
function will be called by the runtime. In the case of shared-memory
we set this section to the function `__wasm_init_memory` which will
initialize all memory on startup.

This also fixes the above mentioned function to ensure we correctly
lower the i32 values.

Lastly, this fixes a typo where we would retrieve a global, instead
of setting its value.