Specialize table access operations for short string keys

[dibyendumajumdar]

This is a common usage in Lua - and we already specialize when the variable is known to be table type (declared as a table) and the key is a short string. But even when the variable is not known to be a table type, if we know that the key is a short string then we could generate a more optimized version of the table get / set operation.

[dibyendumajumdar]

Introduced new bytecodes GETTABLE_SK, SELF_SK and SETTABLE_SK that are emitted when we do not know that the variable is a table - but the key is known to be a short string constant. This covers the many uses of tables. Implementation wise only the interpreter has been enhanced for GETTABLE_SK right now (release 0.17) - work in JIT is still pending. These bytes code default to the standard bytecodes in JIT. Also had to put a workaround in listcode() to emit old opcode names to avoid tests failing.

[dibyendumajumdar]

A related optimisation is to encode the table subtypes in the same way as number, function and string subtypes are defined as 1-2 bit variant flags. I think this will improve performance as currently another field in the structure is inspected to determine whether the type is array type.

[dibyendumajumdar]

Fixed in 0.18 - more work can be done but will be tracked separately