arrow

ARROW-6515: [C++] Clean type_traits.h definitions

The goal of this PR is to uniformize the usage of type traits and pattern matching on static type information.

- Introduce `arrow::enable_if_t` since we're still on C++11. This   removes a small amount of boilerplate, e.g. `typename (expr)::type`. Refactor most code to use this, except where type_traits was not included.
- Major overhaul of `type_traits.h` by exporting a uniformized (as much as I could) aliases, i.e. `is_X_type` and the accompanying `enable_if_X`.
- Removed old struct type traits, e.g. `IsSigned...`.
- Removed catch-all-visitor method and replaced them with explicits missing visitor. This will help the implementation of new types to error missing implementation at compile time instead of runtime.
- Uniformize usage of `enable_if` with methods, by using the return-place form instead of parameter-place (except when in the constructor).
- Fixed some missing implementation when trivial.
- Added c_type to BooleanType

Closes #5885 from fsaintjacques/ARROW-6515-clean-type-traits and squashes the following commits:

41733de54 <François Saint-Jacques> Remove NoExtraMeta
a3d1aea65 <François Saint-Jacques> Revert ARROW_MEMORY_POOL_DEFAULT changes
1a1bf2fb1 <François Saint-Jacques> Review comments
beea0cc34 <François Saint-Jacques> ARROW-6515:  Clean type_traits.h definitions

Authored-by: François Saint-Jacques <fsaintjacques@gmail.com>
Signed-off-by: François Saint-Jacques <fsaintjacques@gmail.com>