There are three minor bugs breaking Go 1.16 with the current version of
garble, after the import path obfuscation refactor:
1) Stripping the runtime results in an unused import error. This PR
fixes that.
2) The asm.txt test seems to be broken; something to do with the export
data not being right for the exported assembly func.
3) The obfuscated build of std fails, since our runtimeRelated table was
generated for Go 1.15, not 1.16.
This PR fixes the first issue, adds conditional skip lines for 1.16 for
the other two issues, and enables 1.16 on CI.
Note that 1.16 support is not here just yet, because of the other two
issues. As such, no doc changes.
Updates #124.