Add AST pass for defaulting PROCs to CDECL
Add optional parameter name removal pass
Add code to solve out unnecessary VERSION nodes
Add AST pass to turn calling conventions into Extern blocks
ast: Rename astAddChild() -> astAppend()
Always emit common callconvs explicitly

instead of covering them by Extern in one version and having them explicit
in another. This produces nicer results, less duplication.
Handle common callconvs also for procptr subtypes, not just procdecls
Move tests into tests/basic/ to make room for #include/AST merging tests
Add some code to allow presets to download and extract tarballs