Want to allow putting structs in libraries and re-using the
libraries.  Current, ugly, solution is to use "alias" keyword at end
of Struct statement.  Eg.,

Struct SomeStruct alias 'xyz'
{
    Text internal;
}

Currently just lexing and parsing for it.  But goal is that eventually
whenever see Struct SomeStruct type in text, will replace with the
aliased text, 'xyz' when emitting.  That way, can inclue other
packages that might have structs defined.
Off-by-one error.
Printing lexer exceptions properly.
Allows importing struct definitions from other files using the alias command.
Added test code for importing structs from other libraries.