optimize reordering of ml-matches to avoid unnecessary computations

This now chooses the optimal SCC set based on the size of lim, which
ensures we can assume this algorithm is now << O(n^2) in all reasonable
cases, even though the algorithm we are using is O(n + e), where e may
require up to n^2 work to compute in the worst case, but should require
only about n*min(lim, log(n)) work in the expected average case.

This also further pre-optimizes quick work (checking for existing
coverage) and delays unnecessary work (computing for *ambig return).

doc/src/manual/methods.md

@@ -322,10 +322,10 @@ julia> g(2.0, 3.0)
 ERROR: MethodError: g(::Float64, ::Float64) is ambiguous.
 
 Candidates:
-  g(x::Float64, y)
-    @ Main none:1
   g(x, y::Float64)
     @ Main none:1
+  g(x::Float64, y)
+    @ Main none:1
 
 Possible fix, define
   g(::Float64, ::Float64)