Disallow absolute file paths in command line

This is a pretty lame “workaround” for #48… but at least there’s no
longer a big exception stacktrace, and I’m not sure what the correct
solution would be anyways.
Better diagnostics for unexpected tokens

Exception message now contains the entire token including its position
in the file.
Fix empty sequences

Empty sequences have a `null` sequencedArgument instead of an empty one.

(Found in test for #48, which is coming in the next commit, so I’m not
adding a separate test case for this.)
Fix #48

BaseMemberExpressions can have type arguments.

Thanks @thradec for reporting!