More consistent evaluation environment

[sunxd3]

The PR brings the following changes to the code:

• An eval_env is created early in the pipeline (right before transformed data are computed, after array sizes are known). eval_env will be used across the entire compilation and logp computation.
• merge_with_coalescence is removed, it is slow and single-purpose
• Some of the code used to assume that, variables not in env are non-data variables, now they need to check if is_resolved
• find_variables_on_lhs is also removed -- the Var interface will be phased out
• some variable namings are made more coherent

[codecov]

Codecov Report

Attention: Patch coverage is 96.61836% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 81.13%. Comparing base (f9979da) to head (4dbee9c).
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/compiler_pass.jl	96.39%	4 Missing ⚠️
src/variable_types.jl	80.00%	2 Missing ⚠️
src/utils.jl	97.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
- Coverage   82.27%   81.13%   -1.14%     
==========================================
  Files          21       21              
  Lines        1991     1945      -46     
==========================================
- Hits         1638     1578      -60     
- Misses        353      367      +14     

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[sunxd3]

Before:

Dataset	Stage	Avg Time	Min Time	Max Time	Memory Usage
lsat	CollectVariables	91.656 ms	88.738 ms	93.081 ms	31.44 MiB
	DataTransformation	198.143 ms	196.257 ms	203.257 ms	169.02 MiB
	NodeFunctions	894.018 ms	856.648 ms	1.162 s	263.36 MiB
inhalers	CollectVariables	53.827 ms	53.067 ms	62.738 ms	18.42 MiB
	DataTransformation	200.013 ms	197.998 ms	203.605 ms	209.34 MiB
	NodeFunctions	539.151 ms	528.337 ms	814.248 ms	182.89 MiB
leuk	CollectVariables	17.154 ms	16.888 ms	21.298 ms	6.26 MiB
	DataTransformation	77.602 ms	74.918 ms	84.727 ms	92.25 MiB
	NodeFunctions	130.587 ms	129.419 ms	134.164 ms	43.82 MiB
epil	CollectVariables	5.750 ms	5.655 ms	12.651 ms	2.10 MiB
	DataTransformation	28.721 ms	26.649 ms	35.697 ms	29.90 MiB
	NodeFunctions	111.960 ms	110.671 ms	120.250 ms	44.31 MiB

After:

Dataset	Stage	Avg Time	Min Time	Max Time	Memory Usage
lsat	CollectVariables	94.551 ms	90.920 ms	104.237 ms	31.44 MiB
	DataTransformation	126.559 ms	124.153 ms	136.073 ms	71.57 MiB
	NodeFunctions	710.050 ms	669.937 ms	866.867 ms	227.15 MiB
inhalers	CollectVariables	54.531 ms	53.561 ms	65.025 ms	18.41 MiB
	DataTransformation	108.058 ms	106.586 ms	118.391 ms	84.91 MiB
	NodeFunctions	492.562 ms	482.581 ms	675.718 ms	179.89 MiB
leuk	CollectVariables	17.755 ms	17.526 ms	22.863 ms	6.56 MiB
	DataTransformation	57.119 ms	55.818 ms	70.786 ms	63.32 MiB
	NodeFunctions	103.667 ms	100.935 ms	106.005 ms	39.02 MiB
epil	CollectVariables	5.870 ms	5.789 ms	10.047 ms	2.09 MiB
	DataTransformation	22.214 ms	21.697 ms	32.339 ms	20.32 MiB
	NodeFunctions	99.995 ms	98.439 ms	110.111 ms	44.69 MiB