Optimize preconditions related to buffer index type

[treiher]

Loosening of field location and size constraints

In the current implementation the field location and maximum field size is restricted by the used buffer index type: Field_Last (Ctx, Fld) <= Types.Bit_Index'Last / 2 and Field_Size (Ctx, Fld) <= Types.Bit_Length'Last / 2. It should be possible to loosen these overly restrictive constraints.

Removing unnecessary precondition in setter functions

The field setter functions check the field location (Field_Last (Ctx, F_Length) <= Types.Bit_Index'Last / 2) as well as the available space in the buffer (Available_Space (Ctx, F_Length) >= Field_Size (Ctx, F_Length)). It should be sufficient to check the latter.