bootstrap:lexer: lexer_back(): fix for injected tokens
bootstrap: union: add back support for external payload

We actually now see inline payload as a tentative feature that may go
away.
bootstrap: support literal integers with `Floating (i.e. 1 instead of 1.0)
lib/n/builtins_string: add All methods
bootstrap:constraint: enum,union: INIT with for_tag is ::init

If variant payload is TBI_TRIVIAL_CTOR
bootstrap:cprinter: init for enum,union even if not `Default_ctor
bootstrap:topdeps: deps must be attributed to generic methods, not the parent
lib/n/builtins: Slicebuf, Stringbuf: Reserve: grow only, if necessary
lib/n/builtins: slices don't own their memory: OK to grow past cnt
lib/n/builtins: Realloc: fix cap computation
lib/builtins: Slicebuf: Rename met Buffer to Buf
t00/init: add test to check complete overwriting in assign of INIT

Checkl for both `Trivial_copy and `Not_return_by_copy.
bootstrap:cprinter: cast assign statements to void in the C code

In C, assignments are not void expressions. We use ?: liberally in the
generated code. And we could end up with something like
	cond ? ( x = 0 ) : ( returns_void(&x) )

This is an error, as the 2 branch expressions are not consistent in
their types.
bootstrap:cprinter: print_generic_linkage: also needed in non-header
t00/fixme16: { {x=0} } initializers are not fully unified (0 remains a literal)
lib/n/builtins: c: fix slice and string literal macros for global constants