feat: add support for const value

[L2jLiga]

Adds support for const keyword

Fixes #275

References: http://json-schema.org/understanding-json-schema/reference/generic.html#id5

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm

[L2jLiga]

@zekth here's the same what hasOf, hasIf and hasConst does, do you see any tricky cases?

[zekth]

Can't the first condition be factorized by :

/"if":{.*"then":{/.test(str)

WDYT?
Also is there any impact on perfs?

[L2jLiga]

I tried run benchmark, seems there no impact almost at all

[L2jLiga]

I joined all regexp into one

[L2jLiga]

Windows 10 20H2 x64, AMD Ryzen 3500, 32gb RAM, NodeJS 14.15.2

master:

FJS creation x 44,960 ops/sec ±0.64% (90 runs sampled)
CJS creation x 132,537 ops/sec ±0.40% (91 runs sampled)
JSON.stringify array x 3,003 ops/sec ±0.46% (90 runs sampled)
fast-json-stringify array x 7,791 ops/sec ±0.35% (94 runs sampled)
compile-json-stringify array x 6,848 ops/sec ±0.67% (86 runs sampled)
JSON.stringify long string x 6,926 ops/sec ±0.53% (88 runs sampled)
fast-json-stringify long string x 6,874 ops/sec ±0.51% (87 runs sampled)
compile-json-stringify long string x 6,910 ops/sec ±0.63% (88 runs sampled)
JSON.stringify short string x 7,482,311 ops/sec ±0.44% (93 runs sampled)
fast-json-stringify short string x 42,011,320 ops/sec ±0.27% (93 runs sampled)
compile-json-stringify short string x 32,998,545 ops/sec ±0.44% (90 runs sampled)
JSON.stringify obj x 1,528,236 ops/sec ±0.39% (91 runs sampled)
fast-json-stringify obj x 7,692,684 ops/sec ±0.35% (93 runs sampled)
compile-json-stringify obj x 17,273,199 ops/sec ±1.01% (94 runs sampled)

this branch:

FJS creation x 47,992 ops/sec ±0.31% (94 runs sampled)
CJS creation x 133,213 ops/sec ±0.24% (93 runs sampled)
JSON.stringify array x 3,002 ops/sec ±0.43% (93 runs sampled)
fast-json-stringify array x 7,500 ops/sec ±0.31% (94 runs sampled)
compile-json-stringify array x 5,986 ops/sec ±0.55% (91 runs sampled)
JSON.stringify long string x 6,981 ops/sec ±0.48% (87 runs sampled)
fast-json-stringify long string x 7,020 ops/sec ±0.40% (89 runs sampled)
compile-json-stringify long string x 6,974 ops/sec ±0.41% (88 runs sampled)
JSON.stringify short string x 7,510,955 ops/sec ±0.33% (94 runs sampled)
fast-json-stringify short string x 42,628,500 ops/sec ±0.32% (93 runs sampled)
compile-json-stringify short string x 35,977,005 ops/sec ±0.34% (95 runs sampled)
JSON.stringify obj x 1,555,885 ops/sec ±0.54% (90 runs sampled)
fast-json-stringify obj x 7,825,478 ops/sec ±0.46% (96 runs sampled)
compile-json-stringify obj x 18,655,018 ops/sec ±0.99% (96 runs sampled)

P.S. I wouldn't really trust this benchmark results, there're lot of things which may accidentically affect them

[zekth]

LGTM

regarding benchmarks, i just wanted to see if there is an impact from looping to regex.