Implement StringScanner for TruffleRuby in pure Ruby

* Fixes #194.
* This is a fresh new implementation from scratch,
  contributed directly to ruby/strscan, under BSD-2-Clause.
  This was implemented using the strscan tests,
  specs from ruby/spec and the documentation (https://docs.ruby-lang.org/en/master/StringScanner.html).
* lib/strscan.rb now handles the loading for all implementations.
* Test on TruffleRuby 33 to ensure it keeps working on older TruffleRuby releases,
  which do not have the necessary Primitives used by this new implementation.

Author: eregon

.github/workflows/ci.yml

@@ -29,6 +29,7 @@ jobs:
           - { os: windows-latest , ruby: mingw }
           - { os: windows-latest , ruby: mswin }
           - { os: ubuntu-latest , ruby: jruby-9.4 }
+          - { os: ubuntu-latest , ruby: truffleruby-33 }
         exclude:
           - { os: macos-latest , ruby: 2.4 }
           - { os: ubuntu-latest , ruby: 2.5 }

Rakefile

@@ -31,7 +31,7 @@ if RUBY_ENGINE == "jruby"
     ext.target_version = '1.8'
     ext.ext_dir = 'ext/jruby'
   end
-  extra_require_path = "ext/jruby/lib"
+  extra_require_path = "lib"
 elsif RUBY_ENGINE == "ruby"
   require 'rake/extensiontask'
   Rake::ExtensionTask.new("strscan") do |x|

ext/jruby/lib/strscan.rb

@@ -2299,6 +2299,4 @@ Init_strscan(void)
     rb_define_method(StringScanner, "fixed_anchor?", strscan_fixed_anchor_p, 0);
 
     rb_define_method(StringScanner, "named_captures", strscan_named_captures, 0);
-
-    rb_require("strscan/strscan");
 }

ext/strscan/strscan.c

@@ -0,0 +1,19 @@
+case RUBY_ENGINE
+when 'ruby'
+  require 'strscan.so'
+  require_relative 'strscan/strscan'
+when 'jruby'
+  require 'strscan.jar'
+  JRuby::Util.load_ext('org.jruby.ext.strscan.StringScannerLibrary')
+  require_relative 'strscan/strscan'
+when 'truffleruby'
+  if RUBY_ENGINE_VERSION.to_i >= 34
+    require 'strscan/truffleruby_strscan'
+    require_relative 'strscan/strscan'
+  else
+    $LOAD_PATH.delete __dir__
+    require 'strscan'
+  end
+else
+  raise "Unknown Ruby: #{RUBY_ENGINE}"
+end