[Bug Fix] Refactoring of Mark and Span, and bug fix (#183)

* 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`.

Author: Paalon

src/YAML.jl

@@ -30,6 +30,8 @@ using StringEncodings
 include("versions.jl")
 include("queue.jl")
 include("buffered_input.jl")
+include("mark.jl")
+include("span.jl")
 include("tokens.jl")
 include("scanner.jl")
 include("events.jl")

src/mark.jl

@@ -0,0 +1,10 @@
+# Position within the document being parsed
+struct Mark
+    index::UInt64
+    line::UInt64
+    column::UInt64
+end
+
+function show(io::IO, mark::Mark)
+    @printf(io, "line %d, column %d", mark.line, mark.column)
+end