Support bit offsets in struct members (IDA)

[mahaloz]

Since #118, it's become clear we do not support structs that have members at bit offsets:

https://github.com/binsync/libbs/blob/ce1491713643c5cd4360ac1d77898e7e0b1c2264/libbs/decompilers/ida/compat.py#L1013

Tasks

To close this issue, these things need to happen

• Change the definition of Struct and StructMember Artifacts to be bit offsets for members
• Update IDA Pro backend to support bits
• Make new issues for each decompiler that does not have easy support for bit offsets in structs