Simplify the lexer by removing BufferedContent and inlining ViewedContent (#1981)

Instead of reading 64 characters at a time into a rolling buffer
as `shiftChar()` is called, we read 8192 characters at a time into
a complete buffer before any `peek()`/`shiftChar()` operations.

test/asm/ff00+c.asm

@@ -1,5 +1,5 @@
 SECTION "test", ROM0[0]
 	ld [ $ff00 + c ], a
-	; 257 spaces exceeds both LEXER_BUF_SIZE (64) and uint8_t limit (255)
+	; 257 spaces exceeds the uint8_t limit (255)
 	ld [ $ff00 +                                                                                                                                                                                                                                                                 c ], a
 	ld [ $ff00                                                                                                                                                                                                                                                                 + c ], a