Add show function to spans (#222)

* Split mark and span from `src/tokens.jl`

* Add iteration and indexing to `Span`.

* Add methods to get start mark and end mark from tokens.

* Replace `token.span.start_mark` and `token.span.end_mark` to
`firstmark(token)` and `lastmark(token)`.

* Fix existing bugs about getting marks from tokens.

* Replace remained expressions.

* Remove methods for `Span`.

* Add a show function to spans.

src/span.jl

@@ -3,3 +3,5 @@ struct Span
     start_mark::Mark
     end_mark::Mark
 end
+
+show(io::IO, span::Span) = print(io, "(line, column) ∈ (", span.start_mark.line, ", ", span.start_mark.column, ")...(", span.end_mark.line, ", ", span.end_mark.column, ")")