Add the ability to add bitfield members via the `StructureBuilder::AddMember` API

What is the feature you'd like to have?
I would like a parameter for the API function StructureBuilder::AddMember that can be used to specify a bitfield size.

Is your feature request related to a problem?
With the newly added bitfield support the way to add them to structures via the API is to call StructureBuilder::AddMemberAtOffset. The problem with this is it requires knowing the byte offset to put the field at. The offset isn't particularly trivial to determine and I would rather rely on the Binary Ninja type backend to figure out the byte offset as I believe its just Clang.

Are any alternative solutions acceptable?
I'm not sure.

Additional Information:
Calculating the byte offset of the next field isn't always obvious due to alignment. For instance consider the following structure:

struct Foobar {
    uint16_t foo;
    uint32_t bar : 23;
    uint8_t baz;
};

Due to the size of bar it is 4 byte aligned and starts at byte offset 4 within Foobar. However if it had the following definition:

struct Foobar {
    uint16_t foo;
    uint32_t bar : 15;
    uint8_t baz;
};

In this case bar starts at byte offset 2 within Foobar.

Start by locating the StructureBuilder::AddMember and StructureBuilder::AddMemberAtOffset API declarations and implementations, then inspect the existing bitfield support. Define the bitfield-size parameter and verify that AddMember can determine offsets for both Foobar examples without requiring callers to supply byte offsets.