align tinyfs with Go 1.16+ io/fs package

[bgould]

Go 1.16 adds a new package io/fs with interfaces that function as an abstraction layer for filesystems. The analogous interfaces in TinyGo/tinyfs are very close to the ones in the io/fs package, so we should implement those "natively" where possible, at least for Go 1.16 users.

It may be worth considering to require Go 1.16 for using tinyfs going forward so that we can avoid cluttering up the codebase with build tags, etc., to account for the missing io/fs package for Go 1.15 and below.

[deadprogram]

Seems like we still need this @bgould ?

[bgould]

Ok ... took a look at this, shouldn't be too bad to get basic fs.FS support added ... I will create a PR soon