Refine read leb in wasm loader of fast interpreter

[wenyongh]

For the wasm loader of fast interpreter, since the leb format has been checked
in the first time traversing, we can skip the check in the second time traversing.
This PR calls read_leb_quick instead of read_leb_xxx in the second time scan to
improve the loader's performance.

[lum1n0us]

LGTM.

Just a thought – perhaps a leb-cache (mapping from leb values to constant values) would be more effective. It could be shared across different running modes.