Fix most deprecations on Julia 0.7

[nalimilan]

The main change is that strings are now === even if they point to different memory areas, so we need to use pointer to check whether they are really the same. object_id is also no longer a meaningful test for strings.

I tried changing the object_id testset to use big floats instead of strings, but it triggers a crash when closing Julia. Reproducer:

@testset "ID check" begin let
    empty!(InternedStrings.pool)

    a = BigFloat(π)
    target_id = objectid(a)

    a = intern(a)
    @test objectid(a) == target_id

    b = BigFloat(π)
    @test objectid(b) != target_id
    b = intern(b)
    @test objectid(b)== target_id

    @test objectid(intern(BigFloat(π))) == target_id

#    use(a,b)
end end

[oxinabox]

The BigFloat thing is:
JuliaLang/julia#26939

There is a testset waiting for them when that is fixed.

InternedStrings.jl/test/all_kinds_of_types.jl

Lines 47 to 62 in 34e9c24

	# Enable when https://github.com/JuliaLang/julia/issues/26939 is fixed
	false && @testset "BigFloat" begin
	let
	pi1 = intern(BigFloat(π))
	@test pi1 == BigFloat(π)
	@test !addr_eq(pi1, BigFloat(π))
	pi2 = intern(BigFloat(π))
	@test addr_eq(pi1, pi2)
	@testset "type inference" begin
	@test pi1 isa BigFloat
	@inferred intern(BigFloat(π))
	end
	end
	end

[oxinabox]

Re: the content of the PR,
I think #13 covered it all?

(It is annoying when you make a PR and someone else has made the same one, I've been bitten by that a few times.)

[nalimilan]

Yeah, it's mostly redundant, but it tackles a few more deprecations. I've rebased it.

[oxinabox]

I am pretty sure you mean to add this to REQUIRE not to test/REQUIRE

[nalimilan]

Indeed. 😄 Fixed.

[codecov-io]

Codecov Report

Merging #15 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #15   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines          20     19    -1     
=====================================
- Hits           20     19    -1

Impacted Files	Coverage Δ
src/InternedStrings.jl	100% <100%> (ø)	⬆️

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[oxinabox]

Thank you, this is excellent.
Maintaining 0.7 compatibility is something I really appreciate it when someone else works out how to do..

[nalimilan]

Could you tag a new version? That would allow CSV to pass its tests on 0.7 after merging JuliaData/CSV.jl#204.