src/print.c: rewrite printstr() to get rid of code duplication

Avoid a narrowing cast in flo_round under MRB_INT64.

Fix a crash bug when GC is ran while creating a proc with env

mrb_proc_new_cfunc_with_env() allocates RProc with RProc::env as NULL
then allocates REnv and sets it to RProc::env of the allocated RProc. If
incremental GC is ran before "allocates REnv and sets it to RProc::env
of the allocated RProc", the allocated RProc's GC status is
"marked" (Black) and the allocated REnv's GC status is
"unmarked" (White). The next incremental GC sweeps the allocated REnv
without re-marking the allocated RProc. Because the RProc is Black and
the REnv is White.

We need to implement write barrier for the case.

We can force to cause the above situation by the following patch:

    diff --git a/src/proc.c b/src/proc.c
    index f98998f..4f4e25c 100644
    --- a/src/proc.c
    +++ b/src/proc.c
    @@ -92,6 +92,7 @@ mrb_proc_new_cfunc_with_env(mrb_state *mrb, mrb_func_t func, mrb_int argc, const
       struct REnv *e;
       int i;

    +  mrb_incremental_gc(mrb);
       p->env = e = env_new(mrb, argc);
       MRB_ENV_UNSHARE_STACK(e);
       e->stack = (mrb_value*)mrb_malloc(mrb, sizeof(mrb_value) * argc);

With this patch, "rake test" causes segmentation fault.

Fix segfault found using afl-fuzz

Fix typo; Replace extensional with extended

Add —-recursive option to automatically update submodules from git mgems

Fix quotes use during load special path gem of gem.

Respect the directory structure of `include`

As mentioned in the README, two files have been put under the `mruby`
directory.

add_dependency doesn't work

Remove unnecessary backticks

Dr Markus Kuhn published in 1999 an article [1] explaining in details
why we shouldn't use the ASCII grave accent (0x60) as a left quotation.

Backticks have been used most notably to produce nice-looking LaTeX
documents but it doesn't seem to be an issue on modern platforms and
for the oldest ones, there are workarounds as mentioned by Dr Kuhn.

[1]: https://www.cl.cam.ac.uk/~mgk25/ucs/quotes.html

provide targets for building native extensions during CrossBuild

Alloc doc

Mrb string value cstr patch

Need mruby-compiler to build mruby-bin-mruby and mruby-bin-mirb for

Proc#curry should preserve lambdas