Document all the extensions in ASLi

[alastairreid]

Documentation goes in the ASLi website.
Extensions are relative to ASL version 1.0 beta 7 (see Arm's DDI0612 beta 7 document).

• Named parameters and default arguments Planned extension: named arguments #29, Planned extension: default arguments #68
• Parameterized records
• Exception markers
• "printf" function (Semi)Planned extension: "printf" #30
• Rename Sat to Saturate
• Halting execution Proposed extension: Prelude functions to halt execution #63
• topslice syntax "-:" Feature: add "-:" bitslice syntax #59
• "expr with { changes }" New feature: syntax to set bits in expression #62

There are also some extensions that are not intended for use in specifications but that are useful in testing or in internal optimizations

• __sint(N)
• __let ... __in ...
• __assert ... __in ...
• syntax for writing bitvectors in hex: 32'xffff_ffff

See also #4 and #5