Java/C#/Python prefixed size support (#4445)

* initial changes to support size prefixed buffers in Java

* add slice equivalent to CSharp ByteBuffer

* resolve TODO for slicing in CSharp code generation

* add newly generated Java and CSharp test sources

* fix typo in comment

* add FinishSizePrefixed methods to CSharp FlatBufferBuilder as well

* add option to allow writing the prefix as well

* generate size-prefixed monster binary as well

* extend JavaTest to test the size prefixed binary as well

* use constants for size prefix length

* fuse common code for getRootAs and getSizePrefixedRootAs

* pulled file identifier out of if

* add FinishSizePrefixed, GetSizePrefixedRootAs support for Python

* Revert "extend JavaTest to test the size prefixed binary as well"

This reverts commit 68be4420dda47e8d0600bb19691f03be71503a68.

* Revert "generate size-prefixed monster binary as well"

This reverts commit 2939516fdf78df4f061c627221e232b312301417.

* fix ByteBuffer.cs Slice() method; add proper CSharp and Java tests

* fix unused parameter

* increment version number

* pulled out generated methods into separate utility class

* pulled out generated methods into separate utility class for Python

* fix indentation

* remove unnecessary comment

* fix newline and copyright

* add ByteBufferUtil to csproj compilation

* hide ByteBuffer's internal data; track offset into parent's array

* test unsafe versions as well; compile and run in debug mode

* clarify help text for size prefix

* move ByteBuffer slicing behavior to subclass

* fix protection levels

* add size prefix support for text generation

* add ByteBufferSlice to csproj compilation

* revert size prefix handling for nested buffers

* use duplicate instead of slice for removing size prefix

* remove slice subclass and use duplicate for removing size prefix

* remove slice specific tests

* remove superfluous command line option