simplify_log() does not

[rwst]

sage: log(8)/log(2)
log(8)/log(2)
sage: _.simplify_log()
log(8)/log(2)
sage: log(8,16)
log(8)/log(16)
sage: _.simplify_full()
log(8)/log(16)
sage: _.simplify_log()
log(8)/log(16)
sage: log(x^3)/log(x)
log(x^3)/log(x)
sage: _.simplify_log()
log(x^3)/log(x)
sage: log(8)/log(2)
log(8)/log(2)
sage: _.simplify_log('all')
log(8^(1/log(2)))

Of course these simple cases should be handled, even if the description of simplify_log defines specific defaults of behaviour. At least simplify_full() should do it by calling some helper.

Component: symbolics

Issue created by migration from https://trac.sagemath.org/ticket/21574