code-gen: add field checks to partials when on staging

[dirkdev98]

This works by dumping the possible keys, and looping over the input objects. It is guarded by an isStaging() check, so shouldn't be too much of a performance hit.

Closes #466

[github-actions]

==== C8 test coverage ====
Statements: 70.15% ( 18291/26075 )
Branches : 71.9% ( 2165 /3011 )
Functions : 70.37% ( 677 /962 )
Lines : 70.15% ( 18291 /26075 )

[github-actions]

==== Benchmark result ====

Node.js version: 15

Total benchmarks: 13
          Passed: 13
          Failed: 0
-----------
lbu validator simple                    2000000  iterations      506  ns/op
yup validator simple                      20000  iterations    60645  ns/op
fastest-validator validator simple     10000000  iterations      140  ns/op
lbu validator nested                      50000  iterations    48642  ns/op
yup validator nested                       5000  iterations   410922  ns/op
fastest-validator validator nested      2000000  iterations      800  ns/op
logger - strings                         500000  iterations     2122  ns/op
logger - objects                         500000  iterations     2674  ns/op
logger - deep objects                    500000  iterations     4154  ns/op
uuid                                    2000000  iterations      528  ns/op
uuid.isValid                           10000000  iterations      148  ns/op
isNil                                 500000000  iterations        0  ns/op
isPlainObject                         100000000  iterations       18  ns/op

[github-actions]

==== Benchmark result ====

Node.js version: 14

Total benchmarks: 13
          Passed: 13
          Failed: 0
-----------
lbu validator simple                    5000000  iterations      454  ns/op
yup validator simple                      20000  iterations    69548  ns/op
fastest-validator validator simple     10000000  iterations      123  ns/op
lbu validator nested                      50000  iterations    46155  ns/op
yup validator nested                       2000  iterations   520699  ns/op
fastest-validator validator nested      2000000  iterations      772  ns/op
logger - strings                         500000  iterations     2262  ns/op
logger - objects                         500000  iterations     2791  ns/op
logger - deep objects                    500000  iterations     4933  ns/op
uuid                                    2000000  iterations      595  ns/op
uuid.isValid                           10000000  iterations      128  ns/op
isNil                                 500000000  iterations        0  ns/op
isPlainObject                         100000000  iterations       18  ns/op

[dirkdev98]

Backends have expected failures.
Pretty cool feature tho, @tjonger !