Added a C code printing method for the sign function.
C code printer now underscores symbols that match reserved words.

This fixes issue #8190 for the C code printer. If the user has a symbol name
that matches a core reserved word in the C language it gets an underscore as a
prefix.
Added a codegen test to check functions with reserved words.
fixed a bug in crypto.rsa_private_key
Implement _sage_ helpers for re/im functions

fixes #4230
Docstring for Expr

fixes #4985
Print Limit arrows with unicode

fixes #5255
Merge pull request #8222 from skirpichev/4230-fix

Implement _sage_ helpers for re/im functions
Added recursive printing for sign and fixed tests.
Moved the reserved words check into CodePrinter so all code printers can use it.
Add test for issue 5719, fixes #5719
Merge pull request #8199 from moorepants/c-sign-function

Added a C code printing method for the sign function.
Added back the deference symbol printing.
Reworked Symbol printers to use super() methods and added test for dereferencing + reserved words.
Merge pull request #8048 from zsc347/master

fixed a bug in crypto.rsa_private_key
Merge pull request #8231 from skirpichev/5719-fix

Add test for issue 5719
Fixed conflict.
Merge pull request #8224 from skirpichev/expr-doc

Docstring for Expr
Binomial parameter checking
Fixed the mutable set issue.