Introduce symbol table API into pad

[happy-barney]

Changes summary

For purpose of this changes let's distinguish between:

• sigil - expression type representation in source code
• symbol type - internal identification of symbol type

For implementation of sigil-less symbols we will still need their symbol type, eg:

• attributes
• named patterns
• data contracts
• ...

replace literals representing type of symbol stored in pad with explicit tokens

For example, '&' in code can mean also:

• subroutine symbol type
• subroutine sigil
• code-block prototype
• predefined variable name

This set of changes replaces its usage as symbol type with C token Perl_Symbol_Table_Code.

Extend padname API with "split" macros

Instead of relying on layout of string content explicit macros providing symbol type and symbol name are introduced, eg:

// current approach:
*PadnamePV (pn)
PadnamePV (pn)[0]

// new approach
Padname_Symbol_Table (pn)

• change allows to change underlying data type and values (eg to index to registry array)
• using explicit token expresses intention and makes it easier to search code base

Provide padname functions with split symbol type and symbol name parameters

Alternatives to existing functions but with two parameters.
Example:

// current
pad_add_name_pvs("$(self)", 0, NULL, NULL);

// new approach
pad_add_symbol_pvs (Perl_Symbol_Table_Scalar, "(self)", 0, NULL, NULL);

Possible follow-ups

• replace more characters literals with macros, eg:
  • Perl_Var_Match
  • Perl_Prototype_Code
• provide builtin symbol table registry
• use this approach also with global symbols

Open questions

• what (if anything) to put into perldelta ?
• mark old functions as deprecated ?

[jkeenan]

What is the point of this very large change? AFAICT, it's not a bug fix or a response to an open issue. Does it respond to some previously expressed need?

[happy-barney]

@jkeenan already discussed that on IRC, this PR is quite huge, it in fact combines parts of 3 changesets (in my eyes, maybe more by someone else)

I will extract first two changes into separated PRs, each announced first for discussion on mailing list.