Implement Struct#values_at and Array#values_at .
Add API `mrb_get_values_at()` to mruby/range.h .
Merge branch 'values_at' of https://github.com/take-cheeze/mruby into take-cheeze-values_at
raise TypeError instead of ArgumentError when argument of wrong type given to the values_at
do not raise error in mrb_range_beg_len()
convert range edges to integers
reindent else
use proper length for mrb_get_values_at()

separate mrb_range_beg_len() into two: the one truncates range into the
sequence size, and the one does not.  #values_at uses the latter.
new macro mrb_int(mrb,x) to retrieve mrb_int from mrb_value with conversion if needed
refactoring mruby-struct
Merge branch 'take-cheeze-values_at'