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Expression evaluation instruction

Expression evaluation using an extended version of Dijkstra's "shunting
yard" algorithm with JavaScript semantics. This algorithm was chosen as
it is simple, sufficient, and can be implemented compactly, minimizing
the size of the code and making it easier to verify correct.

Features:

 - Number, Boolean, Null, and String types
 - String parsing of backslash char, hex, and unicode escapes
   - unicode escapes can contain up to 6 hex characters with a
     value of <= 0x10FFFF
   - "\n", "\x20", "\u2018", "\u01f600" are all valid strings
 - Decimal and hex numbers
 - Assignment statements
 - Variable references
 - Operator precedence
 - Common math, binary and logical operators
 - Both strict and non-strict equality
 - Nesting of sub-expressions with parenthesis
 - Single pass evaluation of multiple semicolon-delimited expressions
 - String equality and concatenation
 - Function calls with a variable number of arguments
   - example: max(1, 2, 3, ..., N) is accepted
 - Type conversion functions
 - Common constants (pi, e, etc)
 - Configurable parse and reduce limits
   - maximum tokens an expression can contain
   - maximum length of a concatenated string

The algorithm assumes the expression is well-formed and performs minimal
validation.  A malformed expression should either terminate evaluation
early or produce no value.

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