Integrate definition of `MRuby::Build#exefile`

In 4174e02, we removed the symbol hash table from `mrb_state` but
`find_symbol` was too slow with linear search. My performance estimation
was wrong. So we implemented a new compact hash table for symbols.

Use linear search instead. Number of symbols is usually small (<1K), so
we don't need performance boost from hash tables. In our benchmark
measurement, hash tables consumes 790KB for `build/full-debug/mrbtest`.

Move `NONE` to `mrblib/enum.rb`

Remove old comment for `mrb_sym2name_len()` [ci skip]

Fix markup and remove unneeded comment for doc in `src/string.c` [ci skip]

Fix `Symbol#size` for multi-byte characters with `MRB_UTF8_STRING`

Before:

  p :あ.size  #=> 3

After:

  p :あ.size  #=> 1

Use `mrb->eException_class` instead of `mrb_class_get()`

Extend only when necessary in `lib/mruby-core-ext.rb`

Remove unneeded `dup` in `Enumerator#initialize`

Remove unused macro in `src/string.c`

`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`