`class`/`module` expression  with empty body should return `nil`

Before:

  p(class A end)          #=> A
  p(class << self; end)   #=> #<Class:#<Object:0x7fdc3880e420>>
  p(module B end)         #=> B

After/Ruby:

  p(class A end)          #=> nil
  p(class << self; end)   #=> nil
  p(module B end)         #=> nil

fix Time about carry-up and carry-down

Fix tests for `String#reverse` with `MRB_UTF8_STRING`

Fix memory leak `sval` when out of memory in `Data_Wrap_Struct()`

Avoid a side effect when run Rake without execution of tasks

Refine error message for time interval

Time interval value can be zero, and float (in `Kernel#sleep`)

Use `assert_raise` and `assert_nothing_raised` in `mruby-sleep` tests

Remove definition of `assert_nothing_raised` in `IO` test

Use assertion methods in `File` test

Remove unused file for `mruby-io` test

Remove definition of `Kernel#sprintf` in `mruby-print`

Remove redundant `NULL` check for `mrb_malloc`

[ci skip] Remove no meaning statement in `bm_app_lc_fizzbuzz.rb`

Remove `.gitkeep` files

Avoid directory creation when run `rake -T` etc.

Remove special treatments for `MRB_WITHOUT_FLOAT` in build scripts

Avoid runtime evaluation for `MRB_WITHOUT_FLOAT`

Fix assertion name for `Numeric#**` test

Fix coercing for first step counter in `Numeric#step`

Before:

  a=[]; 7.step(4, -3.0) { |c| a << c }; p a  #=> [7, 4.0]

After / Ruby:

  a=[]; 7.step(4, -3.0) { |c| a << c }; p a  #=> [7.0, 4.0]

[ci skip] Sort `.gitignore`

Build fails when set `conf.build_dir=<rel path>` and `conf.enable_cxx_exception`