Add WIT support in text parser

Add [WIT](https://github.com/WebAssembly/module-types/blob/master/proposals/module-types/Overview.md) parser. This would be a milestone for future support of WASI #36, since the definition of [WASI](https://github.com/WebAssembly/WASI/blob/master/phases/snapshot/witx/wasi_snapshot_preview1.witx) is written in witx.