Merge pull request #2023 from ksss/khash-readable

Use kh_end instead to n_buckets
Hash#replace should copy default from original even when the default value of the original is not set
Merge branch 'master' of github.com:mruby/mruby
Fix C++ files detection for automatic C++ ABI enabling.
Merge pull request #2025 from take-cheeze/cxx_abi_fix

Fix C++ files detection for automatic C++ ABI enabling.
Add MRuby::Build#disable_cxx_exception to force C++ ABI disable.
Add documentation of MRuby::Build#disable_cxx_exception .
Merge pull request #2026 from take-cheeze/disable_cxx_exception

Add MRuby::Build#disable_cxx_exception to force C++ ABI disable.
Fix wrong `mrb_bug` use found with mruby-clang-plugin.
Fix build_config.rb for ArduinoDue
Merge pull request #2030 from kyab/fix-build_config-arduinodue

Fix build_config.rb for ArduinoDue
Merge pull request #2029 from take-cheeze/mrb_bug_fix

Fix wrong `mrb_bug` use found with mruby-clang-plugin.
mention 1.9 compatibility in README.md; close #2031
Merge branch 'master' of github.com:mruby/mruby
improve hash function based on #2027; close #2027
Make bintest enabling like other `enable_xxx` methods.
Merge pull request #2032 from take-cheeze/enable_bintest

Make bintest enabling like other `enable_xxx` methods.
Add MRB_TT_HAS_BASIC_P macro to check whether a typetag has MRB_OBJECT_HEADER.
Merge pull request #2033 from take-cheeze/mrb_tt_has_basic_p

Add MRB_TT_HAS_BASIC_P macro.
fixed typos in operand descriptions