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ARROW-7775: [Rust] fix: Don't let safe code arbitrarily transmute readers and writers

This code were quite clearly unsafe (which seems to be known, judging
from the comment about it). As the functions appeared to callable
indirectly from other public functions I changed these to just panic
instead to keep the API.

Closes #6256 from Marwes/unsafe_rust and squashes the following commits:

97e022a57 <Markus Westerlind> s/typ/type/g
9fe6ddfd9 <Markus Westerlind> Remove unnecessary Send/Sync impls for RecordBatch
d484e17e3 <Markus Westerlind> Better error message for typed conversion
66ce5b109 <Markus Westerlind> fix: Don't let safe code arbitrarily transmute readers and writers

Authored-by: Markus Westerlind <markus.westerlind@distilnetworks.com>
Signed-off-by: Chao Sun <sunchao@apache.org>