Update opcodes to reflect removal of length-prefixed FlexSym structs

[popematt]

Issue #, if available:

292

Description of changes:

• Describes how to switch a struct from SID field names to FlexSym field names
• Simplifies e-expression opcodes 40-4F to use a bias + trailing FixedUInt
• Reassigns opcodes 50-CF over to 60-DF in order to use 50-5F for e-expressions
• Adds opcode EE for e-expressions with a trailing FlexUInt macro address

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