We don't need to require valid STDIN/STDOUT/STDERR. If we require it,
we can't use mruby on an environment that doesn't have valid
STDIN/STDOUT/STDERR such as Windows service process. Windows service
process doesn't have valid STDIN.

That loads 32 bit integer bypassing pool access.

Reduce recompilation when updating presym target files

Fix dependencies of `y.tab.o`

Fix that updating presym target files would recompile almost all files even
if symbols did not change.

In the case of `y.tab.o`, `y.tab.d` was not read, so `y.tab.c` was not
recompiled by updating header files.

It seemed that the cause was overwriting the rake rule, so I deleted
unnecessary settings.

New instructions:
  * OP_LOADL16
  * OP_LOADSYM16
  * OP_STRING16

Size of pools, symbols are `int16_t` but offset representation in the
bytecode was 8 bits. Size of child `irep` array is `int16_t`, too but
this change does not address it.

`minirake` was still used in many occasions.
This reverts commit 3847c7eb.

Since `R-assignment` in CRuby is abandoned.  Single-line pattern matching
in `mruby` only matches single local variable at the moment. Currently
it works as a right assignment to a local variable. It will be enhanced
in the future.

Fix parallel build

When `rake -m` and so on are used to build in parallel, building may be
started before presym files are generated. Then, for example, the following
error occurs and this issue is fixed.

```console
In file included from /Users/shuujii/mruby/mruby/include/mruby.h:92:
/mruby/mruby/include/mruby/presym.h:16:10: fatal error: '../build/presym.inc' file not found
#include <../build/presym.inc>
         ^~~~~~~~~~~~~~~~~~~~~
```

ossfuzz: Fix mruby include paths

Add `preview` to version.

If you confirm the gem is cdump safe, add `spec.enable_cdump` in
`mrbgem.rake` file. Some external gems e.g. `mruby-mgem-template` do
not work with cdump.

Always use `mrblib` and 'src` for directory names.

Fix typo in `Range#last`