Move jl_init_ to the public header

[gbaraldi]

This is needed for PackageCompiler.jl like workflows

[JeffBezanson]

This could use a better name; maybe jl_init_with_image_data?

[topolarity]

It was intentionally left a bit obscure, since we already have jl_init_with_image_file and jl_init_with_handle

jl_init_with_image_buf seems fine to me (at least it's consistent with the type name)

[topolarity]

Also @gbaraldi why are the other entrypoints insufficient for you?

[topolarity]

@gbaraldi Is this still needed?

[gbaraldi]

Nope