Avoid invalidations by replacing some convert methods by convenience constructors

[fingolfin]

... by convenience constructors. Indeed, from the Julia stdlib, we see
the following convert methods, which imply that these constructors
are fully sufficient:

convert(::Type{T}, x::T)      where {T<:Number} = x
convert(::Type{T}, x::Number) where {T<:Number} = T(x)::T

Note that Julia often inserts convert calls into code automatically, so not adding the above avoids a numerous method invalidations in other packages.

This is progress on issue #278.

[fingolfin]

I was wondering why this did not work as intended in Julia 1.8 compared to 1.9 -- the reason is that in 1.8 they still have convert(::Type{T}, x::Number) where {T<:Number} = T(x) which was improved by adding the type assertion at the end in 1.9/1.10 (i.e., convert(::Type{T}, x::Number) where {T<:Number} = T(x)::T).

I'll see if I can use that to improve the methods here.