Move traits to ProximalCore, new functions, RecursiveArrayToolsExt, defer loading OSQP

[hakkelt]

This PR is part of the effort to make StructuredOptimization as general as possible (see the PR on OperatorCore: JuliaFirstOrder/ProximalCore.jl#5).

Changes:

• PrecomposedSlicedSeparableSum: function to be used for complex problems with multiple variables and variables sliced in different terms. E.g.:

x, y = Variable(10), Variable(5)
problem = ls(x[1:5] - y) + norm(x, 1) + norm(x[6:10], 1)

• reshapeInput wrapper can help, for example, in low-rank minimization to avoid reshaped Eye from AbstractOperators and its unnecessary copy:

x = Variable(3,3,5)
problem = norm(reshape(x, 9, 5), *)

• RecursiveArrayToolsExt extension contains some functions that were moved from StructuredOptimization as I believe they belong here.
• Loading OSQP dependency was delayed until IndPolyhedralOSQP is instantiated. The reason behind it was that OSQP is one of the largest dependencies of ProximalOperators, and it is only used by a single function. ProximalOperators loads almost a second faster without it.

[hakkelt]

I'll break this large PR into multiple smaller ones, but I'll keep this open for reference.