Inline metering with injected gas function

[cdetrio]

Implements the concepts from ewasm/design#182.

[cdetrio]

Injected stuff looks like this:

  (func $useGas (export "useGas") (type $t8) (param $p0 i32)
    (local $l1 i32)
    global.get $gas_global
    local.get $p0
    i32.sub
    local.tee $l1
    i32.const 0
    i32.lt_s
    if $I0
      unreachable
    end
    local.get $l1
    global.set $gas_global)

  (global $gas_global (export "gas_global") (mut i32) (i32.const 2000000000))