Fixing the issues in the tests

[kmp5VT]

Description

This PR seeks to fix the issues pointed out by Matt in the initial commit of this repo.

Fixes #(issue)

I added a extensions for FillArrays and created default type parameters for an AbstractFill.

Checklist:

• Default parameters are defined for AbstractFill and AbstractUnallocatedFill types.
• Tests all run as expected with no errors

[kmp5VT]

@mtfishman It looks like the problem was related to looping over the types written as sybmols. For example in for typ in (:Fill, :Zeros), typ was being interpreted as a symbol and, though we had defined a default_type_parameter for the AbstractFill type, we don't have default parameters defined for symbols which was causing the code to error out.

[mtfishman]

Can you delete the corresponding commented code from test_basics.jl?

[lkdvos]

@mtfishman , I think the easiest would be if you merge this, I'll apply the new skeleton on top of that in a separate PR?