Expose mem allocator on C-API

[mgreter]

Combining #1973 and #1821

Adds the following new C-API functions:

• void* sass_alloc_memory(size_t size)
• char* sass_copy_c_string(const char* str)
• void sass_free_memory(void* ptr)

IMO this should be future proof and both linked issues should be covered.

• Update documentation
• Delete obsolete interface
• Get rid of Sass::sass_strdup

I suggest to add them with #1974 to 3.3.5 as incubating features for 3.4.0.

//CC @xoofx, @am11, @drewwells

[xoofx]

ship it! 😅

[xoofx]

Also a broader plan would be to replace all internal malloc/free so that they go through this API to allocate things (and internally also all C++ objects would use the same allocator), so that custom mem allocator could then be pluggable.

[am11]

LGTM - Thanks @mgreter ! 🎉

[coveralls]

Coverage increased (+1.2%) to 79.184% when pulling 913150f on mgreter:feature/C-API-mem-allocator into aa8aa2d on sass:master.

[xzyfer]

I plan to release 3.3.5 this week. I'd do it sooner but I'm at a conference.

@mgreter can you please get started on some release notes for this change?

[am11]

@mgreter,

Adds the following new C-API functions:
• void* sass_alloc_memory(size_t size)
• char* sass_copy_c_string(const char* str)
• void sass_free_memory(void* ptr)

can we ADDAPIfy these three signatures as well, so they are visible for FFI (P/Invoke)? Currently dumpbin /exports libsass.dll does not list them.

At the moment I am using local patch to make it work:

 // to allocate buffer to be filled
-void* sass_alloc_memory(size_t size);
+ADDAPI void* ADDCALL sass_alloc_memory(size_t size);
 // to allocate a buffer from existing string
-char* sass_copy_c_string(const char* str);
+ADDAPI char* ADDCALL sass_copy_c_string(const char* str);
 // to free overtaken memory when done
-void sass_free_memory(void* ptr);
+ADDAPI void ADDCALL sass_free_memory(void* ptr);