The debug level set by `--debug[=N]` is now available to pure and
startup code as debugLevel, using unsafePerformIO.
`dbg LABEL ...` is now the go-to helper for tracing values on the
console; it produces output when the debug level is non-zero. `dbgExit`
is similar but exits immediately, avoiding further output. The
`dbgshow`, `dbgppshow` and `dbgpprint` variants allow control over the
pretty-printing method and required debug level, allowing more control
over what is displayed when.
Other cleanups: lstrace -> ltrace, pdbgAt -> pdbg, tracewith -> traceWith.
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