deprecate programming interfaces: get, mget, eval

[jangorecki]

Personally I am very firmly after backward compatibility (therefore adding to 2.0.0 milestone), but just noticed quite a list of open issues caused by legacy programming interfaces (get, mget, eval). I doubt below list is complete.

Bugs in special handling of scoping for variables

• weird exception when by contains get #1985
• When column name is name of variable get(variable) generates an error and eval(variable) returnes the incorrect values #4878
• using eval() in DT seems not consistent in terms of external variables #4798
• strange eval failure in by when there is a condition in i #1356
• get breaks .SDcols #2647

Special handling of scoping variable bugs during real/standard use of eval()

• eval doesn't work on join columns #1180
• Eval of quote()-ed expression in wrong frame in := with join #2176

Possibly bug introduced by special handling of eval()

• issue with assignment of eval'd object with by= #1857

[MichaelChirico]

I would be open to deprecating these sooner than 2.0.0, but we definitely need to let the new programming interface mature for some time (e.g. 2-4 releases) before going that route.