Add isconstant field to Constraint; use to skip updating constant constraints

[tkoolen]

Reasons:

• more efficient
• allows a solver that doesn't allow updating certain constraint types to be used as long as the constraint function is constant (ref #92)

(Also some indentation fixes).

[codecov[bot]]

Codecov Report

Merging #93 into master will increase coverage by 0.36%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage    97.9%   98.26%   +0.36%     
==========================================
  Files           9        9              
  Lines         525      635     +110     
==========================================
+ Hits          514      624     +110     
  Misses         11       11

Impacted Files	Coverage Δ
src/functions.jl	99.69% <ø> (+0.44%)	:arrow_up:
src/moi_interop.jl	98.75% <100%> (+0.5%)	:arrow_up:
src/FunctionWrappersQuickFix.jl	69.56% <0%> (-2.66%)	:arrow_down:
src/lazyexpression.jl	100% <0%> (ø)	:arrow_up:
src/util.jl	100% <0%> (ø)	:arrow_up:
src/parameter.jl	100% <0%> (ø)	:arrow_up:
src/debug.jl	100% <0%> (ø)	:arrow_up:
src/model.jl	97.64% <0%> (+1.44%)	:arrow_up:

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