SASyLF version 0.21 - bug fix release

[Jonathan Aldrich]

SASyLF version 0.21 is now available at:

http://www.cs.cmu.edu/~aldrich/SASyLF/SASyLF-0_21.zip

This is a bug-fixing release. Specifically:
* Fixed bug (nested lambdas with the same variable name) by making
internal typechecking more robust
* Fixed bug where a derivation currently being proven by analysis can
be used as if it's true within a subcase
* Fixed bug due to incorrect variable scoping in mutually recursive
theorems

Let me know if there are any issues with this release!

Thanks,

Jonathan