Update eye func: integer(int8)

Author: zoziha

doc/specs/stdlib_linalg.md

@@ -123,11 +123,12 @@ This is an `intent(in)` and `optional` argument.
 
 ### Return value
 
-Return the identity matrix, i.e. a matrix with ones on the main diagonal and zeros elsewhere. The return value is of type `integer`.
+Return the identity matrix, i.e. a matrix with ones on the main diagonal and zeros elsewhere. The return value is of type `integer(int8)`.
+The use of `int8` was suggested to save storage.
 
 #### Warning
 
-Since the result of `eye` is of `integer` type, one should be careful about using it in arithmetic expressions. For example:
+Since the result of `eye` is of `integer(int8)` type, one should be careful about using it in arithmetic expressions. For example:
 ```fortran
 real :: A(:,:)
 !> Be careful

src/stdlib_linalg.fypp

@@ -91,17 +91,17 @@ contains
 
         integer, intent(in) :: dim1
         integer, intent(in), optional :: dim2
-        integer, allocatable :: result(:, :)
+        integer(int8), allocatable :: result(:, :)
 
         integer :: dim2_
         integer :: i
 
         dim2_ = optval(dim2, dim1)
         allocate(result(dim1, dim2_))
 
-        result = 0
+        result = 0_int8
         do i = 1, min(dim1, dim2_)
-            result(i, i) = 1
+            result(i, i) = 1_int8
         end do
 
     end function eye