include, tag: remove bogus const enum declaration

A const enum return value is a tautology. Fixes the following warning:

    src/gumbo.h:361: warning: type qualifiers ignored on function
    return type
parser, tag: remove always true asserts

It's implementation-defined whether an enum with only non-neaative
values is signed or unsigned. If it's unsigned, the `tag >= 0` check
generates a warning.

Fixes the following warnings:

    src/parser.c:682: warning: comparison of unsigned expression >= 0
    is always true
    src/tag.c:179: warning: comparison of unsigned expression >= 0
    is always true
parser: remove dead store

The `node` variable is assigned to but its value is never used. Remove
it altogether. Fixes the following warning:

    src/parser.c: In function 'clear_stack_to_table_body_context':
    src/parser.c:898:14: warning: variable 'node' set but not used
parser: silence 'no return value' warning

The code path should be unreachable so add an abort() statement. Fixes
the following warning:

    src/parser.c:1550: warning: control reaches end of non-void function
char_ref: remove always true assert

size_t is unsigned so a >= 0 check is always true.
Merge pull request #18 from bnoordhuis/compiler-warning-fixups

Compiler warning fixups