RFC: construct the name to object dicts lazily

[mlubin]

Following up on the discussion in jump-dev/JuMP.jl#1402. This change avoids creating the names_to_var dictionary until it's needed.

Using @IainNZ's script that benchmarks variable creation time,

Before:

10^1:  0.000 s, 3981.300000 ns/var, 286 a, 28.600000 a/var
10^2:  0.000 s, 1016.850000 ns/var, 844 a, 8.440000 a/var
10^3:  0.001 s, 703.424000 ns/var, 6756 a, 6.756000 a/var
10^4:  0.007 s, 664.980500 ns/var, 69774 a, 6.977400 a/var
10^5:  0.100 s, 1003.817160 ns/var, 699808 a, 6.998080 a/var
10^6:  1.558 s, 1558.397552 ns/var, 6999859 a, 6.999859 a/var

After:

10^1:  0.000 s, 3638.900000 ns/var, 286 a, 28.600000 a/var
10^2:  0.000 s, 829.310000 ns/var, 838 a, 8.380000 a/var
10^3:  0.001 s, 520.511000 ns/var, 6742 a, 6.742000 a/var
10^4:  0.005 s, 491.230300 ns/var, 69754 a, 6.975400 a/var
10^5:  0.077 s, 766.043240 ns/var, 699776 a, 6.997760 a/var
10^6:  1.005 s, 1004.825593 ns/var, 6999809 a, 6.999809 a/var

so we get the ~0.5 seconds back as predicted.

These results are with an immutable VariableInfo struct (jump-dev/JuMP.jl#1525).

Closes #536

[codecov-io]

Codecov Report

Merging #535 into master will decrease coverage by 0.08%.
The diff coverage is 91.3%.

@@            Coverage Diff             @@
##           master     #535      +/-   ##
==========================================
- Coverage   95.65%   95.56%   -0.09%     
==========================================
  Files          46       46              
  Lines        4692     4712      +20     
==========================================
+ Hits         4488     4503      +15     
- Misses        204      209       +5

Impacted Files	Coverage Δ
src/attributes.jl	95.65% <ø> (ø)	⬆️
src/Utilities/universalfallback.jl	99.3% <ø> (ø)	⬆️
src/Utilities/model.jl	99.57% <100%> (-0.43%)	⬇️
src/Test/modellike.jl	98.06% <33.33%> (-1.94%)	⬇️

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[blegat]

We should update the doc here:
https://github.com/JuliaOpt/MathOptInterface.jl/blob/3b59433e427b5b691cac91c6df5f84ae47d61fea/src/attributes.jl#L160-L168
and say that it now needs to error as discussed in #536

We should also update do here:
https://github.com/JuliaOpt/MathOptInterface.jl/blob/3b59433e427b5b691cac91c6df5f84ae47d61fea/src/attributes.jl#L506
to say when the error is thrown
and here:
https://github.com/JuliaOpt/MathOptInterface.jl/blob/3b59433e427b5b691cac91c6df5f84ae47d61fea/src/attributes.jl#L569