Fix buffer overlow in case method docstring (#1615)
Fix buffer overlow in case method docstring (#1615)

Prior to this commit the `add_docstring` function had a
buffer overlow issue that was causing memory to be
clobbered.

This commit changes the `add_docstring` function to
use a `printbuf` to accumulate the string ensuring
no more buffer overflow issues.

Fixes #1609
Praetonus/ponyc,jemc/ponyc,sgebbie/ponyc,mkfifo/ponyc,boemmels/ponyc,Theodus/ponyc,Praetonus/ponyc,CausalityLtd/ponyc,CausalityLtd/ponyc,jemc/ponyc,cquinn/ponyc,Praetonus/ponyc,mkfifo/ponyc,kulibali/ponyc,sgebbie/ponyc,boemmels/ponyc,sgebbie/ponyc,sgebbie/ponyc,sgebbie/ponyc,jemc/ponyc,ponylang/ponyc,boemmels/ponyc,cquinn/ponyc,kulibali/ponyc,Perelandric/ponyc,ponylang/ponyc,kulibali/ponyc,boemmels/ponyc,ponylang/ponyc,dipinhora/ponyc,dipinhora/ponyc,Perelandric/ponyc,kulibali/ponyc,Theodus/ponyc,mkfifo/ponyc,Theodus/ponyc,Theodus/ponyc,dipinhora/ponyc,mkfifo/ponyc,mkfifo/ponyc,cquinn/ponyc,Perelandric/ponyc,Praetonus/ponyc,Perelandric/ponyc,Theodus/ponyc,CausalityLtd/ponyc,boemmels/ponyc,Perelandric/ponyc,cquinn/ponyc