C++ is pickier about when a 'goto' can cross a variable being delcared.
The fix is to just add a set of braces to restrict the variable's scope.
Without this, g++ will fail with:

  regcomp.c:3057: error: jump to label 'set_call_attr'
  regcomp.c:3087: error:   from here
  regcomp.c:3041: error:   skips initialization of 'int gnum'

I originally solved this by renaming the "node" type to "node_t",
but Matz didn't like that.  He suggested renaming the member
variable "nd" instead:
  https://github.com/mruby/mruby/pull/144#issuecomment-5743153

C++ is picker than C about when you can "goto" across a variable being
defined.  The fix is to just minimize the variable's scope inside an
extra set of brackets.

Without this change, g++ has the following errors:

  transcode.c:590: error: jump to label 'resume_label3'
  transcode.c:514: error:   from here
  transcode.c:582: error:   crosses initialization of 'const unsigned char* p'
  transcode.c:2124: error: jump to label 'set_encs'
  transcode.c:2184: error:   from here
  transcode.c:2088: error:   skips initialization of 'const char* err'
  transcode.c:2089: error:   skips initialization of 'size_t error_len'
  transcode.c:2090: error:   skips initialization of 'mrb_value bytes'
  transcode.c:2091: error:   skips initialization of 'mrb_value dumped'
  transcode.c:2092: error:   skips initialization of 'size_t readagain_len'
  transcode.c:2093: error:   skips initialization of 'mrb_value bytes2'

See Matz's comment here:
  https://github.com/mruby/mruby/pull/144#issuecomment-5754054

mrb_gc_mark_ht_size() and mrb_gc_free_ht() were declared in gc.h as
taking a "RHash *" argument, but then they were defined in hash.c
as taking a "RClass *"  Get these in sync.

see http://en.cppreference.com/w/cpp/language/operator_alternative

Just making this a bit more consistent throughout the code

The following is legal code in both C and C++:

  struct foo {
    struct bar { int a } x;
    int y;
  };

...however in C++ it defines a type called "foo::bar" instead of "bar".
Just avoid this construct altogether

Add Tests for String, Hash, Range and Symbol

Finish all Integer ISO Tests

don't close stdin.

Minor edits to test suite and Math module

Added some tests for Math module. Some refactoring in math.c; numeric tolerance exported as Math::TOLERANCE and also used for fuzzy tests in mrbtest suite