[Rust] Add trait bounds for T in Buffer/List

[asfimport]

This is a first step in refactoring the rust API to make better use of traits and generics (and moving away from enums and macros).

This change introduces a trait for the primitive types that are allowed in Buffer and List and removes some use of macros.

Reporter: Andy Grove / @andygrove
Assignee: Andy Grove / @andygrove

PRs and other links:

• GitHub Pull Request #1966

_{Note: This issue was originally created as ARROW-2528. Please see the migration documentation for further details.}

[asfimport]

Andy Grove / @andygrove:
PR: #1966

[asfimport]

Andy Grove / @andygrove:
Closing this Jira as it will be easier to just go with the full refactor, which we already have a Jira for.