Deprecate functions not supported in SymPyCore

[jverzani]

Add deprecation warnings for move to SymPyCore backend.

Some deprecations have been hard to incorporate and are listed here. None should be so disruptive, save perhaps the first two:

True -> Sym(true)
False -> Sym(false)

sympy_core -> sympy.core
sympy_matrices -> sympy.matrices

@symfuns, @vars

[codecov-commenter]

Codecov Report

Attention: 50 lines in your changes are missing coverage. Please review.

Comparison is base (473447b) 56.15% compared to head (b6dca29) 55.81%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #533      +/-   ##
==========================================
- Coverage   56.15%   55.81%   -0.34%     
==========================================
  Files          20       21       +1     
  Lines        1259     1263       +4     
==========================================
- Hits          707      705       -2     
- Misses        552      558       +6     

Files	Coverage Δ
src/SymPy.jl	62.00% <ø> (ø)
src/constructors.jl	88.88% <100.00%> (+0.20%)	⬆️
src/importexport.jl	23.76% <ø> (+0.94%)	⬆️
src/lambdify.jl	93.42% <100.00%> (ø)
src/sets.jl	77.77% <100.00%> (+6.34%)	⬆️
src/symfunction.jl	70.00% <ø> (ø)
src/utils.jl	78.62% <100.00%> (+0.16%)	⬆️
src/generic.jl	43.75% <50.00%> (ø)
src/permutations.jl	0.00% <0.00%> (ø)
src/plot_recipes.jl	0.00% <0.00%> (ø)
... and 2 more

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