This patch adds dynamic breakpoints to GHCi


There is a new ':breakpoint' command to manage breakpoints.
GHCi simply uses the breakpoint api functions in ghc-api to install itself as a client.
The mechanism used by GHCi to keep track of enabled breakpoints is a simple table.

When a breakpoint is hit, a new interactive session is launched and the bindings in the breakpoint are injected. Some commands are disabled in this sub session