embed small string
use flags 4 for *this object is embed*
use flags 8~64 for *embed string length*
make embed string when create literals
add embed string test
Fix behavior Hash#eql?
.PHONY should be placed after `all'

Because some make (e.g. nmake) recognize it as a normal rule definition and if it's the first rule, make runs it as a default rule.
Consequently `all', `test' and `clean' are all ran (because they are depended by .PHONY), and builded files are cleaned.
Merge pull request #1823 from ksss/hash-eql

Fix behavior Hash#eql?
Hash#== and eql? should not return fixnum; ref #1823
Merge pull request #1824 from unak/patch-1

.PHONY should be placed after `all'
Merge pull request #1820 from ksss/string-embed

Embed small string
reduce integer type complexity from int/size_t/uint32_t by unifying them to uint32_t; close #1778
convert too much from size_t to uint32_t; mrb_sym2name_len() takes size_t*