Adapt TypeError Message for String to Integer Conversion.

MRI:
"str" * "3"
=> TypeError: no implicit conversion of String into Integer

mruby:
"str" * "3"
=> TypeError: String can't be coerced into int
Adapt for Float and Integer
use suitable type
Use CRuby compatible key in ObjectSpace.count_objects .
Merge pull request #1953 from bovi/conversion-str-int

Adapt TypeError Message for String to (Integer|Float) Conversion
Merge pull request #1954 from take-cheeze/objectspace_count_key

Use CRuby compatible key in ObjectSpace.count_objects .
Implement String#hex
Merge pull request #1956 from cubicdaiya/issues/suitable_type5

Use size_t instead of int
Merge pull request #1959 from mattn/string_hex

Implement String#hex
allow underscores in integer literals; close #1960
an int literal may have multiple underscores inside