Move jerry-core API implementations and headers into dedicated subdirectories (#1793)

Moved all public API headers under the `jerry-core/include`
directory. This makes installing all the public headers easier.
Also, should we have new public headers in the future, their
installation will be automatic, there will be no need to update the
build files. Moreover, this aligns better with the structure of
other libraries in the project (in those cases, public headers
always reside in `<library>/include`).

Moved all public API implementations under the `jerry-core/api`
directory. This cleans up the root directory of `jerry-core`,
moving all implementation code under "modules", i.e.,
subdirectories. This also makes the future splitting of the big and
monolithic `jerry.c` along features easier, if needed. (Debugger
and snapshot-related functions are already in separate sources.)

Notes:
* `jerryscript.h` is split up to separate header files along
  feature boundaries. These new headers are included by
  `jerryscript.h`, so this is not a breaking change but header
  modularization only.
* `jerry-snapshot.h` is still under `jerry-core/api`, keeping it as
  a non-public header.
* This commit also adapts all targets to the include path change.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

targets/mbed/source/jerry_extapi.cpp

@@ -16,7 +16,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 
-#include "jerry-core/jerryscript.h"
+#include "jerry-core/include/jerryscript.h"
 #include "jerry_extapi.h"
 
 #include "native_mbed.h"
@@ -99,7 +99,7 @@ register_native_function (const char* name,
   jerry_value_t global_object_val = jerry_get_global_object ();
   jerry_value_t reg_function = jerry_create_external_function (handler);
 
-  bool is_ok = true; 
+  bool is_ok = true;
 
   if (!(jerry_value_is_function (reg_function)
         && jerry_value_is_constructor (reg_function)))