-bloogle-db: Bloogle suggestions in unbound-name errors and typed-hole warnings

[matx-amy]

Stacked on #969 and #1110.

Adds a -bloogle-db <db> flag. When it's given, bsc runs bloogle search <db> ... (with bloogle from PATH) for some of its diagnostics and appends the results.

Every unbound-name error (variables, constructors, fields, type constructors, classes, packages) gets the exact-name matches, which mostly answers "which package do I import for this?":

package Q(mkQ) where

mkQ :: Module Empty
mkQ = module
    q <- mkFIFO
    rules
        when True ==> q.enq (0 :: Bit 8)

Error: "Q.bs", line 5, column 9: (T0004)
  Unbound variable `mkFIFO'
  bloogle found these exact name matches:
    FIFO mkFIFO :: (Bits a sa, IsModule m c) => (m (FIFO a))

and it works the same for e.g. an unbound type constructor:

Error: "T.bs", line 3, column 15: (T0007)
  Unbound type constructor `FIFO'
  bloogle found these exact name matches:
    module FIFO
    FIFO data FIFO a

Typed holes (#1110) get the approximate matches from a type search on the inferred type, so the foldr __ 0 v example from that PR becomes:

Warning: "Sum.bs", line 6, column 15: (T0159)
  Typed hole `__' has the inferred type:
    Prelude.Bit 8 -> Prelude.Bit 8 -> Prelude.Bit 8
  bloogle found these approximate matches:
    Prelude min :: (a -> a -> a)
    Prelude max :: (a -> a -> a)
    Prelude (&) :: (a -> a -> a)
    Prelude (^) :: (a -> a -> a)
    Prelude (~^) :: (a -> a -> a)

The database is the one you'd build for Bloogle itself, e.g. bloogle generate bluespec.hoo $BLUESPECDIR/bloogle from the .txt that #969 installs -- so it can include your own libraries too, not just the standard ones. Without the flag nothing changes at all; with it, a missing or failing bloogle just leaves the diagnostics undecorated.

Since the unbound-name errors are raised in several phases (MakeSymTab, kind inference, typecheck, GenSign), the lookup lives in the common reporting funnel, bsWarningsAndErrorsWithContext, rather than at each raise site: setErrorHandleFlags stashes the database path in the ErrorState, and a wrapper (EBloogleMatches) appends the matches to the wrapped message's text while keeping its tag. Unbound type variables, exports, and $tasks are deliberately not looked up, since a library search can't help with those. The tests use a stub bloogle script, so they don't depend on a real install.

Known gap: the type query is the hole's pretty-printed type, so for a BSV file it's in BSV syntax (UInt#(8)), which Bloogle doesn't parse, and those holes get unhelpful approximate matches. Classic holes and all the name lookups are unaffected.