[2992970] Restore the safety of Tcl_AppendObjToObj(x, x) for bytearrays.
Also moves overflow checking to TclAppendBytesToByteArray() and adds
the ability to call TABTBA() with bytes==NULL, for appending unspecified
bytes.  That is, the string grows, but the new bytes are of undetermined
value.  Like Tcl_NewByteArrayObj(NULL, length) this option is useful
for manipulating buffers.  The TABTBA growth algorithm is also enhanced
a bit, copying over a fuller implementation from GrowStringBuffer()
in tclStringObj.c
added compilation for [nextto]
implementation of [yieldto] in bytecode
[2992970] Restore safety of Tcl_AppendObjToObj(x, x) for bytearrays.
Silence compiler warnings.
merge mark
[a90d9331bc]: must not crash when yieldto called in vanishing namespace
add compilation for [nextto] and [yieldto]; fix [a90d9331bc]
minor tidying up
remove unused variable
improved tidy-up from branch
merge trunk