introduce wasi.Errno type and switch error codes to constants

[achille-roussel]

This PR implements the changes discussed in #64

I modified the original proposal a bit so that the Error method will return the name of the error code rather than the numeric code, the intent being to be more meaningful when reading formatted errors in application logs.

Let me know if you have any comments!

resolves #64

[mathetake]

LGTM. Thank you @achille-roussel for your contribution!🏅

[codefromthecrypt]

fyi we had to revert this to ErrnoName because all errnos are not errors (ex errnosuccess isn't an error), plus we have some thinking to do wrt how to allow host functions to raise errors #258. Finally, the type alias complicates things like reflection, and generation which are easier if only use go types

ps if you are using something besides tinygo and/or need a WASI API not defined here, please pipe up in the issue #271