WebAssembly control flow instructions

WebAssembly control flow instructions.

block: Creates a label that can later be branched out of with a br.
br: Branches to a loop, block, or if.
br_if: Branches to a loop, block, or if, based on a boolean condition.
br_table: Branches to different loop, block, or if statements, based on an argument.
call: Calls a function.
drop: Pops a value from the stack, and discards it.
end: Can be used to end a block, loop, if, or else.
if...else: Executes a statement if the last item on the stack is true (non-zero).
loop: Creates a label that can later be branched to with a br.
nop: Does nothing.
return: Returns from a function.
select: Selects one of its first two operands based on a boolean condition.
unreachable: Denotes a point in code that should not be reachable.