Replace Rule#char_at with idiomatic Crystal

src/markd/rule.cr

@@ -105,10 +105,6 @@ module Markd
     # accepts_line
     abstract def accepts_lines? : Bool
 
-    private def char_at(parser : Parser, index = parser.next_nonspace) : Char?
-      parser.line[index]?
-    end
-
     private def space_or_tab?(char : Char?) : Bool
       [' ', '\t'].includes?(char)
     end

src/markd/rules/block_quote.cr

@@ -36,14 +36,14 @@ module Markd::Rule
     end
 
     private def match?(parser)
-      !parser.indented && char_at(parser) == '>'
+      !parser.indented && parser.line[parser.next_nonspace]? == '>'
     end
 
     private def seek(parser : Parser)
       parser.advance_next_nonspace
       parser.advance_offset(1, false)
 
-      if space_or_tab?(char_at(parser, parser.offset))
+      if space_or_tab?(parser.line[parser.offset]?)
         parser.advance_offset(1, true)
       end
     end

src/markd/rules/code_block.cr

@@ -52,7 +52,7 @@ module Markd::Rule
         else
           # skip optional spaces of fence offset
           index = container.fence_offset
-          while index > 0 && space_or_tab?(char_at(parser, parser.offset))
+          while index > 0 && space_or_tab?(parser.line[parser.offset]?)
             parser.advance_offset(1, true)
             index -= 1
           end

src/markd/rules/html_block.cr

@@ -3,7 +3,7 @@ module Markd::Rule
     include Rule
 
     def match(parser : Parser, container : Node)
-      if !parser.indented && char_at(parser) == '<'
+      if !parser.indented && parser.line[parser.next_nonspace]? == '<'
         text = parser.line[parser.next_nonspace..-1]
         block_type_size = Rule::HTML_BLOCK_OPEN.size - 1
 

src/markd/rules/list.cr

@@ -98,7 +98,7 @@ module Markd::Rule
         end
       end
 
-      next_char = char_at(parser, parser.next_nonspace + first_match_size)
+      next_char = parser.line[parser.next_nonspace + first_match_size]?
       unless next_char.nil? || space_or_tab?(next_char)
         return empty_data
       end
@@ -115,19 +115,19 @@ module Markd::Rule
 
       loop do
         parser.advance_offset(1, true)
-        next_char = char_at(parser, parser.offset)
+        next_char = parser.line[parser.offset]?
 
         break unless parser.column - spaces_start_column < 5 && space_or_tab?(next_char)
       end
 
-      blank_item = char_at(parser, parser.offset).nil?
+      blank_item = parser.line[parser.offset]?.nil?
       spaces_after_marker = parser.column - spaces_start_column
       if spaces_after_marker >= 5 || spaces_after_marker < 1 || blank_item
         data["padding"] = first_match_size + 1
         parser.column = spaces_start_column
         parser.offset = spaces_start_offset
 
-        parser.advance_offset(1, true) if space_or_tab?(char_at(parser, parser.offset))
+        parser.advance_offset(1, true) if space_or_tab?(parser.line[parser.offset]?)
       else
         data["padding"] = first_match_size + spaces_after_marker
       end