Optimization of REXML::Parsers::SAX2Parser#get_namespace

## Benchmark
```
                         before       after  before(YJIT)  after(YJIT)
                 sax     1.377k      2.247k        1.668k       2.829k i/s -     100.000 times in 0.072609s 0.044501s 0.059965s 0.035352s

Comparison:
                              sax
         after(YJIT):      2828.7 i/s
               after:      2247.1 i/s - 1.26x  slower
        before(YJIT):      1667.6 i/s - 1.70x  slower
              before:      1377.2 i/s - 2.05x  slower
```

- YJIT=ON : 1.70x faster
- YJIT=OFF : 1.63x faster

Author: naitoh

benchmark/sax.yaml

@@ -0,0 +1,34 @@
+loop_count: 100
+contexts:
+  - gems:
+      rexml: 3.2.6
+    require: false
+    prelude: require 'rexml'
+  - name: master
+    prelude: |
+      $LOAD_PATH.unshift(File.expand_path("lib"))
+      require 'rexml'
+  - name: 3.2.6(YJIT)
+    gems:
+      rexml: 3.2.6
+    require: false
+    prelude: |
+      require 'rexml'
+      RubyVM::YJIT.enable
+  - name: master(YJIT)
+    prelude: |
+      $LOAD_PATH.unshift(File.expand_path("lib"))
+      require 'rexml'
+      RubyVM::YJIT.enable
+
+prelude: |
+  require 'rexml/parsers/sax2parser'
+
+  DEPTH = 100
+  xml   = '<a>' * DEPTH + '</a>' * DEPTH
+
+benchmark:
+  'sax'       : |
+    parser = REXML::Parsers::SAX2Parser.new(xml)
+    parser.listen(:start_element){|uri, localname, qname, attrs|}
+    parser.parse

lib/rexml/parsers/sax2parser.rb

@@ -12,7 +12,7 @@ def initialize source
         @parser = BaseParser.new(source)
         @listeners = []
         @procs = []
-        @namespace_stack = []
+        @namespace_stack = [{}]
         @has_listeners = false
         @tag_stack = []
         @entities = {}
@@ -122,7 +122,7 @@ def parse
               event[2].each {|n, v| event[2][n] = @parser.normalize(v)}
               nsdecl = event[2].find_all { |n, value| n =~ /^xmlns(:|$)/ }
               nsdecl.collect! { |n, value| [ n[6..-1], value ] }
-              @namespace_stack.push({})
+              @namespace_stack.push(@namespace_stack[-1].dup)
               nsdecl.each do |n,v|
                 @namespace_stack[-1][n] = v
                 # notify observers of namespaces
@@ -260,10 +260,8 @@ def add( pair )
 
       def get_namespace( prefix )
         return nil if @namespace_stack.empty?
-
-        uris = (@namespace_stack.find_all { |ns| not ns[prefix].nil? }) ||
-          (@namespace_stack.find { |ns| not ns[nil].nil? })
-        uris[-1][prefix] unless uris.nil? or 0 == uris.size
+        current_namespace = @namespace_stack[-1]
+        current_namespace[prefix] || current_namespace[nil]
       end
     end
   end