[refactor] Move Context to taichi/program

[k-ye]

Related issue = #1739

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[codecov]

Codecov Report

Merging #1788 into master will decrease coverage by 0.10%.
The diff coverage is 36.36%.

@@            Coverage Diff             @@
##           master    #1788      +/-   ##
==========================================
- Coverage   42.61%   42.50%   -0.11%     
==========================================
  Files          44       44              
  Lines        6183     6194      +11     
  Branches     1071     1073       +2     
==========================================
- Hits         2635     2633       -2     
- Misses       3394     3406      +12     
- Partials      154      155       +1     

Impacted Files	Coverage Δ
python/taichi/lang/expr.py	60.63% <16.66%> (-1.32%)	⬇️
python/taichi/lang/matrix.py	64.34% <16.66%> (-0.39%)	⬇️
python/taichi/lang/snode.py	64.22% <57.14%> (-1.61%)	⬇️
python/taichi/lang/ops.py	43.56% <66.66%> (-0.33%)	⬇️
python/taichi/testing.py	75.75% <0.00%> (-2.37%)	⬇️

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[yuanming-hu]

Cool! LGTM. Thanks.