Support Bitstr Expression

[yoshihiro503]

• Add Bitstr variant to Ast.constant
• Translate <<X/string, Y/binary>> -> like binary:app(list_to_bin(X), Y)

[yuezato]

References:

• http://erlang.org/euc/07/papers/1700Gustafsson.pdf
• http://erlang.org/doc/programming_examples/bit_syntax.html
• http://www.nct9.ne.jp/m_hiroi/func/abcerl07.html
• http://merrigrove.blogspot.com/2014/01/gotcha-with-bitstrings-in-erlang-naked.html

[yuezato]

A = <<(B=C):(C=2)>>.
* 1: variable 'C' is unbound

and

A = <<(B=1):(C=B)>>.
<<1:1>>

These two suggest that, for <<exp1:exp2>>, exp1 should be evaluated before exp2.

However,

A = <<(B=1):(if (B == 3) -> 5; true -> 6 end)>>.
* 1: variable 'B' is unbound