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

cross compiles

From the CRuby 2.2.2 Proc#arity documentation:

If the block has optional arguments, returns -n-1, where n is the number
of mandatory arguments, with the exception for blocks that are not
lambdas and have only a finite number of optional arguments; in this
latter case, returns n.

Add :mgem gem loading.

Fix repository link to mruby/mruby

Fix typos in documentation and error messages

[ci skip] Remove some Srting#split samples