Ensure float are not implemented as numeric

Ensure float are not implemented as numeric

Fixed the base class for dialect-specific float/double types; Oracle
:class:`_oracle.BINARY_DOUBLE` now subclasses :class:`_sqltypes.Double`,
and internal types for :class:`_sqltypes.Float` for asyncpg and pg8000 now
correctly subclass :class:`_sqltypes.Float`.

Added suite tests to ensure that floating point types, such as
class:`_types.Float` and :class:`_types.Double` are not resolved as
class:`_types.Numeric` in the dialect, since it may not compatible in
all cases, such as when casting a value.

Change-Id: I20b814e8e029d57921d9728a55f2570f74c35c87