Generate lit-magic-strings.inc.h (#1690)

Currently, `lit-magic-strings.inc.h` is manually maintained. This
has several drawbacks:

- keeping the list of magic strings sorted first by length then
  alphabetically is error prone,
- it is easy to leave unused magic strings in the list by accident
  (e.g., `LIT_MAGIC_STRING_JERRY_UL` is defined as a magic string
  but not used anywhere in the code) and,
- it is very hard to add `CONFIG_DISABLE_*_BUILTIN` guards to the
  list, even though there are several magic strings, which are used
  in some of the configurations only (e.g, "setPrototypeOf" is used
  in ES2015 only).

To ease the maintenance of magic strings, this commit moves the
definition of magic strings to a config file
(`lit-magic-strings.ini`), and adds `tools/gen-magic-strings.py` to
generate the `.inc.h` file from this config file and from the use
cases of the strings in the code.

- The magic strings in the config file can appear in any order, the
  generator will ensure that they are correctly sorted.
- The generator skips those definitions that are not used anywhere
  (and emits a warning to signal that such definitions can be
  removed).
- The generator applies the same guards to the definitions in the
  `.inc.h` file as found in the code around the use of the strings
  to optimize for size.

The commit also changes some builtin-related `.inc.h` files by
adding guards that don't affect functionality but improve the
results of the generator.

To ensure that the invocation of the generator does not get
forgotten, the commit also adds `tools/check-magic-strings.sh` and
binds it into the testing infrastructure.

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

docs/04.INTERNALS.md

@@ -261,7 +261,7 @@ Several references to single allocated number are not supported. Each reference
 
 ### String
 
-Strings in JerryScript are not just character sequences, but can hold numbers and so-called magic ids too. For common character sequences there is a table in the read only memory that contains magic id and character sequence pairs. If a string is already in this table, the magic id of its string is stored, not the character sequence itself. Using numbers speeds up the property access. These techniques save memory.
+Strings in JerryScript are not just character sequences, but can hold numbers and so-called magic ids too. For common character sequences (defined in `./jerry-core/lit/lit-magic-strings.ini`) there is a table in the read only memory that contains magic id and character sequence pairs. If a string is already in this table, the magic id of its string is stored, not the character sequence itself. Using numbers speeds up the property access. These techniques save memory.
 
 ### Object / Lexical Environment