Additional convenience macros for flatbuffer schema definition

[quinnj]

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Tracking issue for a few extra convenience macros that would make flatbuffer schema translation a little more straightforward:

• [x] @enumtype T UInt8: since Enum in Julia is Int32 by default and flatbuffers allows declaring an enums size
• [ ] @table type Foo ...: would detect if any union fields are included in the table and add the additional union_type field, as well as provide additional constructors to detect the union_type automatically
• [x] @struct immutable Bar ...: would detect any additional padding fields needed to ensure the struct is aligned according to the largest field member size

[quinnj]

We could also potentially subsume the @default macro into the @table macro to allow declaring defaults inline with the table definition.

[quinnj]

Also as a breadcrumb, after a conversation with @yuyichao, I'm pretty sure we can just declare these macros inside the package itself and export instead of needing them in the src/header.jl file to be included.

[rjkat]

In the flatc branch I have coopted the Parameters.jl way of specifying defaults inline.