This commit replaces "unknown type" errors in getLLVMType with panics.
The main reason this is done is that it simplifies the code *a lot*.
Many `if err != nil` lines were there just because of type information.
Additionally, simply panicking is probably a better approach as the only
way this error can be produced is either with big new language features
or a serious compiler bug. Panicking is probably a better way to handle
this error anyway.
Support for channels is not complete. The following pieces are missing:
* Channels with values bigger than int. An int in TinyGo can always
contain at least a pointer, so pointers are okay to send.
* Buffered channels.
* The select statement.
