Simplify containsconstant trait

[jishnub]

Remove the dependency on SimpleTraits, as for a binary trait we may dispatch on Val(true) and Val(false) directly.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +9.57 :tada:

Comparison is base (1a9877e) 61.84% compared to head (f9738f3) 71.42%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #492 +/- ## ========================================== + Coverage 61.84% 71.42% +9.57% ========================================== Files 81 81 Lines 8513 8520 +7 ========================================== + Hits 5265 6085 +820 + Misses 3248 2435 -813 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/492?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/ApproxFunBase.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/492?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL0FwcHJveEZ1bkJhc2Uuamw=) | `84.00% <ø> (+8.00%)` | :arrow_up: | | [src/Spaces/ConstantSpace.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/492?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL1NwYWNlcy9Db25zdGFudFNwYWNlLmps) | `59.09% <100.00%> (+20.77%)` | :arrow_up: | ... and [55 files with indirect coverage changes](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/492/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation)

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