Added string interpolation helper routines in parserutils
Added a proof-of-concept string interpolation user-land macros (currently, only as a test case):
the interpolated expression could either be transformed to
concat("literal string ", $(interpolated), " end") or
"literal string $1 end" % [$(interpolated)]
Added a very initial definition of Optional[T] generic type
A new overload of ParseIdent was added in hope to get around the fact that the
old one doesn't work correctly in macros, but the problem persists.
Constant expressions used in when statements and tuple indexing are properly evaluated now
Even further step tested to be OK:
```nimrod
proc semConstExpr(c: PContext, n: PNode): PNode =
result = semAndEvalConstExpr(c, n)
```
proc parseExpr*(s: string) : expr {.magic: "ParseExprToAst".}
## Compiles the passed string to its AST representation
## Expects a single expression
proc parseStmt*(s: string) : stmt {.magic: "ParseStmtToAst".}
## Compiles the passed string to its AST representation
## Expects one or more statements
proc getAst*(macroOrTemplate: expr): expr {.magic: "ExpandMacroToAst".}
## Obtains the AST nodes returned from a macro or template invocation
## example:
## macro FooMacro() =
## var ast = getAst(BarTemplate())
Handling of the node.toYaml magic moved to the evaluation engine.
This patch greatly improves the "step over" operation available in debuggers.
In practice, there are often 4-8 lines of C code generated for each nimrod line
Each such line will be responsible to a single step in the debugger that is
a) not expected by the user
b) taking the user to an incorrect line in the nimrod code
To keep this working, all code generation should use the rope formatting
facilities when producing new lines (i.e. $n and $N).
New semantics for the format string are introduced:
$n means "soft new line" that could be joined/broken when lineDir is enabled.
$N means "hard new line" that will always appear as a new line.
As an alternative to this approach, I also tested producing code like this:
#line "code.nim" 154
foo = bar; \
foo(bar) \
This is better for readability of the final output, but unfortunately it didn't
produce the desired result across all compilers/debuggers.
