Atomic keys introduction for strings, symbols, small numbers.

[VadimZhestikov]

Proposed changes

(This PR is clone of unintentionally closed PR 777)

This is series of commits which introduce internal atomic representation for
strings, symbols and small numbers as keys for access object properties.

These commits are intended to speed up access to object properties.

Step 1:
Remove long strings from njs core to simplify code and make it more regular.
Reserve atom_id field in njs_value_t.

Notes:
a) Each commit is fully and successfully tested for regressions with all local tests;
b) This PR is draft for internal review, I'll extend it by additional commits as
they will be merged here from my hg sandbox.
c) As soon as last commit will be added/reviewed, draft status from this PR can be removed,
and PR applied then.

Checklist

Before creating a PR, run through this checklist and mark each as complete:

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes

[VadimZhestikov]

Added njs_cnst.[ch]. It is intermediate commit, the hash itself and unique pre-atomized njs_cnst.vs._STRING_NAMES will be used in next commits.

[xeioex]

Suggested commit log:

Added predefined strings hash.

See my patches that address issues/improvements found by me here https://github.com/xeioex/njs/tree/atomic_strings

applied

[xeioex]

See the patch for the mentioned issues/improvements here 965d68f

comments addressed

[VadimZhestikov]

squashed working commits to simplify history

[VadimZhestikov]

force-push missed change for njs_global_this_prop_handler()

[VadimZhestikov]

polished a bit change for njs_global_this_prop_handler()

[VadimZhestikov]

It is incremental patch.

[VadimZhestikov]

small reordering applied

[VadimZhestikov]

Now atom_ids are used in the code. All local tests are passed. To be squashed later, I'll add optional stuff soon (additional optimizations, randomized atom_ids, etc)

[VadimZhestikov]

Series of working commits are squashed to simplify history. Need take a look at still missed inline optimizations and replacing 'njs_value_t name' by atom_id in njs_object_prop_t.

[VadimZhestikov]

Now performance is same as in my initial sandbox. Looking at replacing 'njs_value_t name' by atom_id in njs_object_prop_t.