Separate IOSource#ensure_buffer from IOSource#match. (#118)

## Why?

It would affect performance to do a read check in `IOSource#match` every
time, Separate read processing from `IOSource#ensure_buffer`.

Use `IOSource#ensure_buffer` in the following cases where
`@source.buffer` is empty.

1. at the start of pull_event
2. If a trailing `'>'` pattern matches, as in `@source.match(/\s*>/um)`.

## Benchmark

```
RUBYLIB= BUNDLER_ORIG_RUBYLIB= /Users/naitoh/.rbenv/versions/3.3.0/bin/ruby -v -S benchmark-driver /Users/naitoh/ghq/github.com/naitoh/rexml/benchmark/parse.yaml
ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [arm64-darwin22]
Calculating -------------------------------------
                         before       after  before(YJIT)  after(YJIT)
                 dom     10.278      10.986        16.430       16.941 i/s -     100.000 times in 9.729858s 9.102574s 6.086579s 5.902885s
                 sax     30.166      30.496        49.851       51.596 i/s -     100.000 times in 3.315008s 3.279069s 2.005961s 1.938123s
                pull     35.459      36.380        60.266       63.134 i/s -     100.000 times in 2.820181s 2.748745s 1.659301s 1.583928s
              stream     33.762      34.636        55.173       55.859 i/s -     100.000 times in 2.961948s 2.887131s 1.812485s 1.790218s

Comparison:
                              dom
         after(YJIT):        16.9 i/s
        before(YJIT):        16.4 i/s - 1.03x  slower
               after:        11.0 i/s - 1.54x  slower
              before:        10.3 i/s - 1.65x  slower

                              sax
         after(YJIT):        51.6 i/s
        before(YJIT):        49.9 i/s - 1.04x  slower
               after:        30.5 i/s - 1.69x  slower
              before:        30.2 i/s - 1.71x  slower

                             pull
         after(YJIT):        63.1 i/s
        before(YJIT):        60.3 i/s - 1.05x  slower
               after:        36.4 i/s - 1.74x  slower
              before:        35.5 i/s - 1.78x  slower

                           stream
         after(YJIT):        55.9 i/s
        before(YJIT):        55.2 i/s - 1.01x  slower
               after:        34.6 i/s - 1.61x  slower
              before:        33.8 i/s - 1.65x  slower

```

- YJIT=ON : 1.01x - 1.05x faster
- YJIT=OFF : 1.01x - 1.06x faster

lib/rexml/parsers/baseparser.rb

@@ -210,6 +210,8 @@ def pull_event
         return @stack.shift if @stack.size > 0
         #STDERR.puts @source.encoding
         #STDERR.puts "BUFFER = #{@source.buffer.inspect}"
+
+        @source.ensure_buffer
         if @document_status == nil
           start_position = @source.position
           if @source.match("<?", true)
@@ -236,6 +238,7 @@ def pull_event
               elsif @source.match(/\s*>/um, true)
                 id = [nil, nil, nil]
                 @document_status = :after_doctype
+                @source.ensure_buffer
               else
                 id = parse_id(base_error_message,
                               accept_external_id: true,
@@ -248,6 +251,7 @@ def pull_event
                   @document_status = :in_doctype
                 elsif @source.match(/\s*>/um, true)
                   @document_status = :after_doctype
+                  @source.ensure_buffer
                 else
                   message = "#{base_error_message}: garbage after external ID"
                   raise REXML::ParseException.new(message, @source)
@@ -646,6 +650,7 @@ def parse_attributes(prefixes, curr_ns)
               raise REXML::ParseException.new(message, @source)
             end
             unless scanner.scan(/.*#{Regexp.escape(quote)}/um)
+              @source.ensure_buffer
               match_data = @source.match(/^(.*?)(\/)?>/um, true)
               if match_data
                 scanner << "/" if closed

lib/rexml/source.rb

@@ -68,6 +68,9 @@ def encoding=(enc)
     def read
     end
 
+    def ensure_buffer
+    end
+
     def match(pattern, cons=false)
       if cons
         @scanner.scan(pattern).nil? ? nil : @scanner
@@ -165,11 +168,14 @@ def read
       end
     end
 
+    def ensure_buffer
+      read if @scanner.eos? && @source
+    end
+
     # Note: When specifying a string for 'pattern', it must not include '>' except in the following formats:
     # - ">"
     # - "XXX>" (X is any string excluding '>')
     def match( pattern, cons=false )
-      read if @scanner.eos? && @source
       while true
         if cons
           md = @scanner.scan(pattern)