Add local mode and nonlocal field tag

[ivan4th]

Some of the fields may only be needed for over-the-network transfer,
but can be skipped when encoding the object locally. For instance, if
the object is stored in a blob field in the database, but some parts
of it also are stored in other columns in the same row, we can save
some space by omitting these fields in the blobs.

This is achieved by adding nonlocal field tag, WithEncodeLocal()
encoder option and WithDecodeLocal() decoder option. The encoder and
the decoder skip fields with nonlocal tag when they're in the local
mode.

[codecov]

Codecov Report

❌ Patch coverage is 38.46154% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.8%. Comparing base (4956740) to head (9f809b7).
⚠️ Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
generate.go	22.2%	12 Missing and 2 partials ⚠️
decoder.go	0.0%	7 Missing ⚠️
tag.go	72.7%	4 Missing and 2 partials ⚠️
encoder.go	0.0%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #92     +/-   ##
========================================
+ Coverage    50.0%   50.8%   +0.7%     
========================================
  Files           6       6             
  Lines         891     929     +38     
========================================
+ Hits          446     472     +26     
- Misses        376     385      +9     
- Partials       69      72      +3     

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