code-gen: inline basic validators

[dirkdev98]

For now we almost always inline references, and string or number oneOf types.

Closes #508

[github-actions]

==== C8 test coverage ====
Statements: 70.64% ( 18005/25488 )
Branches : 71.56% ( 2083 /2911 )
Functions : 70.17% ( 647 /922 )
Lines : 70.64% ( 18005 /25488 )

[github-actions]

==== Benchmark result ====

Node.js version: 14
Total benchmarks: 13
          Passed: 13
          Failed: 0
-----------
lbu validator simple                    5000000  iterations      442  ns/op
yup validator simple                     100000  iterations    11692  ns/op
fastest-validator validator simple     10000000  iterations      116  ns/op
lbu validator nested                      50000  iterations    41619  ns/op
yup validator nested                      10000  iterations   168400  ns/op
fastest-validator validator nested      2000000  iterations      822  ns/op
logger - strings                         500000  iterations     2191  ns/op
logger - objects                         500000  iterations     2669  ns/op
logger - deep objects                    500000  iterations     4608  ns/op
uuid                                    2000000  iterations      526  ns/op
uuid.isValid                           10000000  iterations      123  ns/op
isNil                                 500000000  iterations        0  ns/op
isPlainObject                         100000000  iterations       16  ns/op

[github-actions]

==== Benchmark result ====

Node.js version: 15
Total benchmarks: 13
          Passed: 13
          Failed: 0
-----------
lbu validator simple                    5000000  iterations      496  ns/op
yup validator simple                     100000  iterations    12701  ns/op
fastest-validator validator simple     10000000  iterations      136  ns/op
lbu validator nested                      50000  iterations    46460  ns/op
yup validator nested                      10000  iterations   171476  ns/op
fastest-validator validator nested      2000000  iterations      865  ns/op
logger - strings                         500000  iterations     2407  ns/op
logger - objects                         500000  iterations     2702  ns/op
logger - deep objects                    500000  iterations     4138  ns/op
uuid                                    2000000  iterations      614  ns/op
uuid.isValid                           10000000  iterations      173  ns/op
isNil                                 500000000  iterations        0  ns/op
isPlainObject                         100000000  iterations       19  ns/op