Add TrapezoidalLayout

[mleprovost]

Hello,

I open this issue to discuss the implementation of TrapezoidalLayout, see (https://discourse.julialang.org/t/view-of-lowermatrix-is-not-working-with-ldiv-and-mul/44046/2). This will be helpful to perform ldiv!, mul! on views of triangular matrices for instance.

[dlfivefifty]

An important component will be to make sure the code works for both TrapezoidalLayout and TriangularLayout, when appropriate. Perhaps a design like this makes sense:

abstract type AbstractTrapezoidalLayout{UPLO,UNIT} <: MemoryLayout end
struct TrapezoidalLayout{UPLO,UNIT} <: AbstractTrapezoidalLayout{UPLO,UNIT} end
struct TriangularLayout{UPLO,UNIT} <: AbstractTrapezoidalLayout{UPLO,UNIT} end