tiledb-rs error styling part 1: Context::capi_call returns its own error type

[rroelke]

In tiledb-tables, we've begun to declare at least one Error enum per module. If a module foo calls a function from a module bar, then foo::Error has an enum variant approximating Bar(bar::Error). This can help build a causality chain in error output.

We have discussed bringing this error style into tiledb-rs, and this pull request is a foray into doing so. We begin with the core of tiledb-rs, Context::capi_call. This pull request adds CApiError which is returned by Context::capi_call. The rest of the changes fell out of that in some way.

There are a few // SAFETY comments for new unwraps. These are added in situations where we try to convert a value returned from tiledb into one of the tiledb-rs enums. The presupposition is that if libtiledb returned TILEDB_OK then it must have given a valid enum value. I am personally comfortable with this assumption (I believe I am aligned with this blog about unwrap) but I'm also fine changing it if requested - it would nudge the code in the direction of more Error variants.