sql: introduce PG-compatible access privilege inquiry functions

Fixes cockroachdb#22734.
Fixes cockroachdb#20784.
Relates to cockroachdb#15441.

This change introduces a series of Postgres-compatible privilege-related
builtin functions:
- `has_any_column_privilege`
- `has_column_privilege`
- `has_database_privilege`
- `has_foreign_data_wrapper_privilege`
- `has_function_privilege`
- `has_language_privilege`
- `has_schema_privilege`
- `has_sequence_privilege`
- `has_server_privilege`
- `has_table_privilege`
- `has_tablespace_privilege`
- `has_type_privilege`
- `pg_has_role` (_coming soon!_)

These all follow the specification documented by Postgres in:
https://www.postgresql.org/docs/8.4/static/functions-info.html#FUNCTIONS-INFO-ACCESS-TABLE

These Access Privilege Inquiry Functions allow users to query object
access privileges programmatically. Each function has a number of
variants, which differ based on their function signatures. These
signatures have the following structure:
```
- optional "user" argument
  - if used, can be a STRING or an OID type
  - if not used, current_user is assumed
- series of one or more object specifier arguments
  - each can accept multiple types
- a "privilege" argument
  - must be a STRING
  - parsed as a comma-separated list of privilege
```

This means that in total, each function has at least 6 variants.

The main reason for adding these builtins in is because they were the
last remaining issue that was blocking full compatibility with pgweb.
Pgweb is a web-based database browser written in Go, which means that
can run on OSX, Linux and Windows machines!

Release note (sql change): Introduces a series of Postgres-compatible
privilege-related builtin functions.