Avoid type promotion in increment/decrement

[dmbates]

An operation on an integer array, mat, like

mat .+ 1

will promote the eltype of the result to Int64. A safer way of doing this so as to avoid needless promotion of types is

mat .+ true

because true the Bool value will always be promoted to eltype(mat).

An in-place increment/decrement operation can safely be written as

mat .+= 1

because it is in-place

While making these changes I also changed the Project.toml file to allow later versions of CategoricalArrays and added [extras] and [targets] sections which can replace test/Project.toml. Finally I rewrote the make_testvalues function in test/index.jl to use a comprehension.