Remove usage of isalpha and isdigit from JSON built-in's implementation

[ruben-ayrapetyan]

Hello.

Currently, implementation of JSON built-in uses isalpha and isdigit functions.
Instead, helpers from lit_char should be used.

After the replacement is performed, isalpha and isdigit functions should be removed.

[akosthekiss]

There is a name clash with isalpha if compiler-default library is used for the build (on x86_64 Ubuntu 14.04.2 LTS with gcc 4.8.4). Is it worth reporting/fixing the issue (only 4 lines of changes) or should I wait for the replacement?

[zherczeg]

Fix: PR #484

[akosthekiss]

Fix landed, closing issue