issues uncovered by toysh blog test cases

[andychu]

Divergences:

• evaluation order of >bad echo {undef?err}
• $((echo hello)) somehow parses but it shouldn't
• two failing IFS test cases in spec/toysh (these cases found bugs in yash!)

Not implemented:

• @Q on arrays -- not implemented (crashes)
• extended globs of the file system (fnmatch of extended globs is implemented)

Known differences:

• Brace expansion and tilde sub (OSH agrees with mksh, not bash) - https://www.oilshell.org/release/0.8.pre6/doc/known-differences.html#tilde-expansion-and-brace-expansion-dont-interact
• setting readonly var -- bash is the odd man out
• readonly and temp binding -- this is where shells diverge the most, see huge table by Koichi in unset should unshadow variables higher on the stack (at least for nonlocals) #706