Support `(un)whiten` and `(inv)quad` with static arrays

[devmotion]

Based on #181, this PR also adds support for whiten, unwhiten, quad, and invquad with static arrays.

The PR grew a bit larger, hence I kept it separate from #181 for now.

Main points:

• Do not use mutation in non-mutating methods (in a few cases, allocations could be reduced by specializing on Array, see my comment in #181 - should we do this?)
• Improve composability by forwarding calls with generic matrices to f(..., AbstractPDMat(x), ...) (as a nice side effect, this means that more efficient implementations are called automatically whenever AbstractPDMat(x) is specialized; this is the case, e.g., for Diagonal matrices).
• Check dimensions.

[codecov-commenter]

Codecov Report

Attention: 64 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
src/pdiagmat.jl	99.19% <100.00%> (+1.71%)	:arrow_up:
src/utils.jl	60.00% <100.00%> (-31.67%)	:arrow_down:
src/generics.jl	76.47% <75.00%> (-10.71%)	:arrow_down:
src/scalmat.jl	90.09% <75.00%> (-7.24%)	:arrow_down:
src/pdsparsemat.jl	71.96% <48.83%> (-23.93%)	:arrow_down:
src/pdmat.jl	74.38% <48.27%> (-24.04%)	:arrow_down:

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