hush: faster/smaller code to check for presense of multiple chars in string

hush: faster/smaller code to check for presense of multiple chars in string

Go over the string only once.

function                                             old     new   delta
encode_then_expand_string                            126     105     -21
encode_then_expand_vararg                            443     399     -44
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(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-65)             Total: -65 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>