Fix Base.elsize(<:Type{<:MArray})

[vyu]

Currently, Base.elsize(<:Type{<:MArray}) is not correct for eltypes for which sizeof is not a multiple of Base.datatype_alignment. This leads to pointer(::MArray, ::Int) giving incorrect addresses. For example:

julia> using StaticArrays

julia> primitive type Test24 24 end

julia> Test24(n) = Base.trunc_int(Test24, n);

julia> a = Test24.(1:4);

julia> m = MVector{4}(a)
4-element MVector{4, Test24} with indices SOneTo(4):
 Test24(0x000001)
 Test24(0x000002)
 Test24(0x000003)
 Test24(0x000004)

julia> GC.@preserve m unsafe_load.(pointer.(Ref(m), 1:4))
4-element Vector{Test24}:
 Test24(0x000001)
 Test24(0x0002be)
 Test24(0x030000)
 Test24(0x000000)

This PR fixes this by calling Base.elsize on the corresponding Vector type. (This issue was previously present in Base JuliaLang/julia#33283.)

[thchr]

LGTM.