Always unbox single-constructor datatypes

[melsman]

Always unbox single-constructor datatypes even when the constructor argument is itself unboxed. For instance, consider the datatype declaration

datatype t = N of int

Such types appear in the code generated by ML Yacc and ML Lex. We can also support more general cases such as

datatype ValEnv = VALENV of Ident.Map.map
datatype Env = ENV of StrEnv * ValEnv  
         and StrEnv = STRENV of Env StrId.Map.map

Here all the types (ValEnv, Env, StrEnv) can be represented unboxed even when maps (Ident.Map.map and StrId.Map.map) are represented unboxed (e.g., patricia trees or association lists).

The decision about unboxing is done in src/Compiler/Lambda/CompileDec.sml. The major issue resolved here is for polymorphic equality elimination to generate proper code for comparing singular datatypes for equality. Previously, unnecessary branches were generated, which lead to code being generated for switch-constructs that would null out the least-significant bits in values carried by singular unboxed value constructors.

This PR partly resolves issue #99.