Since in mruby, Integer and Float interchange frequently (mostly on
overflow), so adding explicit `.0` can cause problems sometimes.

For example:
https://github.com/mattn/mruby-json/pull/40
https://github.com/pepabo/mruby-msd/pull/13
https://github.com/mattn/mruby-json/pull/42

Increase symbol bit size on 32-bit mode with `MRB_WORD_BOXING`

This change increases the max length of lower inline symbols to 5 bytes.

Avoid bit fields in `mrb_value`; ref b2c3d88f

The changes at b2c3d88f were inappropriate because the memory layout of bit
fields are implementation defined. Therefor, I fixed it not to use bit
fields.

Merge pull request #4691 from shuujii/fix-Range-max-test-TypeError-is-raised-on-32-bit-mode-with-word-boxing

Fix `Range#max` test (`TypeError` is raised) on 32-bit mode with word boxing

Add `assert_not_nil` method

Fix argument specs to `BasicObject`

Add to `doc/limitations.md` about `nil?` redefinition; ref 49967097 [ci skip]

Merge pull request #4683 from shuujii/enumeratorChain-rewind-shouldnt-rewind-elements-arent-iterated

`Enumerator::Chain#rewind` shouldn't rewind elements aren't iterated

### Example:

  ```ruby
  # example.rb
  e = [1]
  def e.rewind; p :r end
  c = e.chain(e)
  c.each{break c}.rewind
  ```

#### Before this patch:

  ```terminal
  $ bin/mruby example.rb
  :r
  :r
  ```

#### After this patch (same as Ruby):

  ```terminal
  $ bin/mruby example.rb
  :r
  ```

`Array#permutation` with a negative argument should not yield

Before this patch:

  $ bin/mruby -e '[1].permutation(-1){|v| p v}'  #=> [1]

After this patch (same as Ruby):

  $ bin/mruby -e '[1].permutation(-1){|v| p v}'  #=> no output

Compile `if expr.nil?` to use `OP_JMPNIL` instead of calls.

Implement `mrb_immediate_p` macro for `MRB_WORD_BOXING`

The default implementation of `mrb_immediate_p` uses `mrb_type`. However,
in `MRB_WORD_BOXING`, `mrb_type` has many branches and is inefficient, so
provide an implementation that does not use `mrb_type`.

Merge pull request #4677 from shuujii/array-permutation-combination-without-block-should-return-self

`Array#(permutation|combination)` without block should return `self`

Remove unneeded `/src/y.tab.c` in `.gitignore`

Currently, `y.tab.c` is generated to under the `build/` directory.

Fix build of `mruby-random` on 32-bit mode

Fix `Array#sample` with `MRB_INT32`

Array index became potentially negative because `uint32_t` is cast to
`mrb_int`.

Fix wrong argument for `fprintf` in `mruby-bin-mrbc`

Remove unused `random.h`

Fixes for Doxygen docs

Defer several build libraries loading until needed