Implement missing R7RS functions

[jcubic]

• char-foldcase
• environment ==> use %make-env
• exact-integer-sqrt
• list-set!
• string-copy!
• string-downcase
• string-foldcase
• string-for-each
• string-upcase
• vector-copy
• vector-copy!
• vector-for-each
• emergency-exit ==> (exit) + make it work for Browser
• write-shared (handle cycles)
• write-simple (ignore cycles)

Not for version 1.0:

• raise-continuable
• delay-force
• include-ci